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Playing with Apertures

 
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scitch



Joined: 29 May 2010
Posts: 461

PostPosted: Sat Jul 24, 2010 10:46 am    Post subject: Playing with Apertures Reply with quote

As I mentioned earlier, I was doing some experiments for my own knowledge on how aperture affects an image. So, I used a carpenter bee as the subject and a Raynox 250 on a Tamron 90mm on a Sony a200 to do two stacks. One was at F2.8 and the other at F16.

Here is the F2.8, ISO 100, 1/250th second, pop-up flash, 38 images:


Here's a cropped version of the image above:


Here is the F16, ISO 100, 1/250th second, pop-up flash, 18 images:


Here is a cropped version of the F16 image:


Here it is without the Raynox at F5.6 ISO-100, 1/3 second:


Here is that image cropped:



A couple of questions:
I had imagined that if I focused on a subject and then snapped on the Raynox, I'd see the same image, only larger. But what it appears that it does is allow me the same magnification, but from farther away. I can see how that would be valuable in the field. Is that what it's supposed to do or am I doing something wrong? When I snap it on, it's OOF and I have to move the camera back to get it in focus and that reduces the magnification.

So, it seems as though I got better results with the F16 when cropped. It was also easier to light and required fewer images in the stack. So, what's the advantage of a lower aperture?

Also, how should I have handled this particular subject with the dark black head and the bright white shoulders?

Mike
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rjlittlefield
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Joined: 01 Aug 2006
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Location: Richland, Washington State, USA

PostPosted: Sat Jul 24, 2010 10:05 pm    Post subject: Re: Playing with Apertures Reply with quote

scitch wrote:
I had imagined that if I focused on a subject and then snapped on the Raynox, I'd see the same image, only larger. But what it appears that it does is allow me the same magnification, but from farther away. I can see how that would be valuable in the field. Is that what it's supposed to do or am I doing something wrong?

Something is wrong.

In the images posted here, the bee occupies more of the frame when you are using the Raynox than it does when you are not. In other words, the images with Raynox are higher magnification than the ones without.

I suggest checking your images and distances again. I don't have a Raynox to confirm with, but theory says that adding the Raynox should give you slightly more magnification at slightly less distance, measured from the subject to the lens. All this assumes that you do not change the focus setting on your camera's lens.

If you're so inclined, you can readjust focus so as to get the same magnification with and without the Raynox. In that case you might also get more working distance with the Raynox than without it. Exactly how that works depends on details of the lenses that I don't know.

Quote:
When I snap it on, it's OOF and I have to move the camera back to get it in focus and that reduces the magnification.

This sounds strange. Having the image go OOF is normal, but having to move the camera back is not. Normally the camera would have to move forward because the Raynox acts like a magnifying glass -- it takes a close subject and makes it appear to be larger but farther away. I wonder if the focus ring on the main lens is getting moved when you snap on the Raynox.

Quote:
So, it seems as though I got better results with the F16 when cropped. It was also easier to light and required fewer images in the stack. So, what's the advantage of a lower aperture?

Not much in this case, and certainly not as wide as f/2.8.

At this magnification & with this equipment, there's likely to be some improvement in sharpness at f/11, maybe a little more at f/8, but beyond that things will get fuzzier again.

What happens is that there is a tradeoff between diffraction and lens aberrations. At very narrow apertures the image gets fuzzy from diffraction. At very wide apertures it gets fuzzy from aberrations. Best sharpness is someplace in the middle.

Apertures wider than the sharpest setting generally have no advantages unless you count fuzzier backgrounds as being an advantage. Apertures narrower than the sharpest setting, but still sharp enough to look good, have the advantage of allowing shorter stacks and giving a smoother transition from the in-focus part of the stack into the OOF background.

Quote:
Also, how should I have handled this particular subject with the dark black head and the bright white shoulders?

Broader illumination. You want the light hitting the subject from the largest possible range of angles. A light tent that completely surrounds the specimen is good. If you have only one flash, then setting some aluminum foil as a reflector to bounce light from the other direction will help. Look back through NikonUser's series of postings for illustration of how he uses styrofoam beverage cups as diffusers and reflectors. Examples include HERE and HERE.

--Rik
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rjlittlefield
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Joined: 01 Aug 2006
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PostPosted: Sat Jul 24, 2010 10:47 pm    Post subject: Reply with quote

It occurs to me that a bit more background information on the Raynox lens may be helpful.

The big value of lenses like the Raynox 250 is when they are used in conjunction with telephoto lenses that by themselves will not focus at close distances.

For example, a 200 mm lens that will normally focus only to 3 feet will give only about 0.2X magnification, but when used in conjunction with the Raynox 250, it will focus at a few inches and will give almost 2X magnification.

The added value is less when the Raynox is used with shorter lenses that are designed to focus close by themselves. This is the reason that you are not seeing big effects using the Raynox in conjunction with a 100 mm macro lens.

--Rik
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scitch



Joined: 29 May 2010
Posts: 461

PostPosted: Sat Jul 24, 2010 11:11 pm    Post subject: Reply with quote

"In the images posted here, the bee occupies more of the frame when you are using the Raynox than it does when you are not. In other words, the images with Raynox are higher magnification than the ones without. "

Sorry, those images were cropped a little too. There was a lot of blank space to the left and right that I cropped off because there was nothing valuable there. That's why the images are square instead of rectangular. So, don't take the field of view in these images to mean anything. Looking back at the original images, the field of view was nearly identical, but remember that I had to move the tripod to make it that way.

"I suggest checking your images and distances again. I don't have a Raynox to confirm with, but theory says that adding the Raynox should give you slightly more magnification at slightly less distance, measured from the subject to the lens. All this assumes that you do not change the focus setting on your camera's lens. "

I will check them again as this was my first time. When I snapped on the Raynox, I was unable to focus the image without moving the tripod. So, I had to move it back to focus it and apparently, that canceled out the increased magnification.

"This sounds strange. Having the image go OOF is normal, but having to move the camera back is not. Normally the camera would have to move forward because the Raynox acts like a magnifying glass -- it takes a close subject and makes it appear to be larger but farther away. I wonder if the focus ring on the main lens is getting moved when you snap on the Raynox."

I thought it was strange also. It is possible, but not likely that the focus ring is moving. The Raynox doesn't require any pressure to snap on. It has two spring loaded clips that get squeezed and pressed on lightly. I will be careful to pay attention to this next time. I may have moved it in determining that it was beyond the lens's ability to focus.

"At this magnification & with this equipment, there's likely to be some improvement in sharpness at f/11, maybe a little more at f/8"

I knew that I was going to the extremes just to see what the difference was. The default setting was 5.6 or 6.5, I don't remember for sure. So, I'll keep in mind to stay between 6 and 11. At 2.8, I had to diffuse the flash so much just to keep the picture from burning out.

I will keep playing with lighting and diffusion. I'm learning that it is extremely important and is different with each subject. I tried some natural lighting outdoors today with subjects that were still alive. That was even more challenging. I don't have an external flash yet, just the tiny little built in one.

I'll post some of those pictures shortly because I want to know if any of the spiders are dangerous for my family's sake. I also want to make absolutely sure that what I think are hoverflies are not actually bees.

Mike
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ChrisLilley



Joined: 01 May 2010
Posts: 680
Location: Nice, France (I'm British)

PostPosted: Sun Jul 25, 2010 5:48 am    Post subject: Re: Playing with Apertures Reply with quote

scitch wrote:
As I mentioned earlier, I was doing some experiments for my own knowledge on how aperture affects an image. So, I used a carpenter bee as the subject and a Raynox 250 on a Tamron 90mm on a Sony a200 to do two stacks. One was at F2.8 and the other at F16.

I had imagined that if I focused on a subject and then snapped on the Raynox, I'd see the same image, only larger.


A close-up lens is not designed to work like that, no. It is not a snap-on "add magnification to current image" gadget.

It does change the magnification, but it also changes both the closest focus distance (much shorter) and the furthest focus distance (also shorter, i.e no longer at infinity). It also changes the focal length of the combination (lens+close-up lens).

Don't focus without the close-up lens then add the close-up. Treat the combination as a different lens with different properties.

The Raynox DCR-250 is a +8.0 diopter lens. With the Raynox fitted, your 90mm lens has a focal length of 52mm when focused at infinity. (I don't know if the focal length of the lens gets shorter when it focuses closer; many do, and that would affect the focal length of the combination when focused closer also).

Guessing that you are using the Tamron SP AF 90mm f/2.8 DI macro, whose spec sheet is here the closest focus distance of the lens is 0.29m (measured from the sensor in the camera, not from the front of the lens).

With the Raynox fitted, the new closest focus distance is 0.087m (87mm!). Recall that this distance is measured from the sensor; the Sony A200 measures 44.6 mm from sensor to lens mounting flange, and the SP AF90mm has a length of 97mm. A bit of that length is the mount, which goes inside the camera, but even so the distance from the sensor to the front of the lens is going to be around 135mm. So if the lens was set to the minimum focus distance and the Raynox DSC-250 was added, the new closest focus distance would be well inside the lens.
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ChrisLilley



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Location: Nice, France (I'm British)

PostPosted: Mon Jul 26, 2010 6:58 am    Post subject: Reply with quote

During yesterdays forum outage, Rik emailed me to point out that my calculations above make some invalid assumptions ("thin lens" assumptions). While my general point that you can't snap on a close up lens and retain focus still stands, the calculated minimum focus distance in particular is wrong.

I hope Rik posts what he sent me, it was instructive.
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scitch



Joined: 29 May 2010
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PostPosted: Mon Jul 26, 2010 10:13 am    Post subject: Reply with quote

Thanks Chris and Rik. I actually got the Raynox for my Casio Exilim because the lens does not come off. But I couldn't help sticking it on the macro lens either. The Casio can record HD video and over 1,000 frames per second, but doesn't do macro well.

I'll keep playing and keep learning.

Mike
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rjlittlefield
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PostPosted: Mon Jul 26, 2010 12:16 pm    Post subject: Reply with quote

Quote:
During yesterdays forum outage, Rik emailed me to point out that my calculations above make some invalid assumptions ("thin lens" assumptions). While my general point that you can't snap on a close up lens and retain focus still stands, the calculated minimum focus distance in particular is wrong.

I hope Rik posts what he sent me, it was instructive.

Sure. Here it is, with a bit of cleanup...

ChrisLilley wrote:
The Raynox DCR-250 is a +8.0 diopter lens. With the Raynox fitted, your 90mm lens has a focal length of 52mm when focused at infinity. (I don't know if the focal length of the lens gets shorter when it focuses closer; many do, and that would affect the focal length of the combination when focused closer also).

Guessing that you are using the Tamron SP AF 90mm f/2.8 DI macro, whose spec sheet is here the closest focus distance of the lens is 0.29m (measured from the sensor in the camera, not from the front of the lens).

With the Raynox fitted, the new closest focus distance is 0.087m (87mm!). Recall that this distance is measured from the sensor; the Sony A200 measures 44.6 mm from sensor to lens mounting flange, and the SP AF90mm has a length of 97mm. A bit of that length is the mount, which goes inside the camera, but even so the distance from the sensor to the front of the lens is going to be around 135mm. So if the lens was set to the minimum focus distance and the Raynox DSC-250 was added, the new closest focus distance would be well inside the lens.

Sorry, but this analysis is not correct. It sounds like you're treating the lenses as "thin" and not separated. When they are thick and separated, as in reality, the behavior is quite different. Even within the thin-lens model, your calculation of "new closest focus distance" has gone astray. For a thin lens, the minimum focus distance as measured from sensor occurs at 1:1, at which point the lens is 2 focal lengths away from the sensor and another 2 focal lengths from the subject -- 4 focal lengths altogether. For a 52 mm focal length, that would be 208 mm from sensor, not 87 mm as you write.

As a different way of thinking about the same problem, imagine that you have a "thin" close-up lens while the Tamron is whatever it is. Set the Tamron to its closest focus distance. Obviously it focuses at a point well in front of the lens. Now, place the imaginary thin close-up lens at that same distance, so that the subject and the thin lens are coincident. In this configuration, the close-up lens has no effect on focus. Starting from that configuration, slide the close-up lens closer to the Tamron. As you do that, the focus point will gradually move closer to the Tamron, but it will remain on the outer side of the close-up lens, away from the camera. This relationship persists until you reach the limit of movement when the close-up lens touches the Tamron.

This is probably a good time to put in another plug for WinLens, a ray-tracing program that I have found very helpful for understanding & confirming the behavior of lens combinations. The cool thing about WinLens is that because it's ray-tracing and shows you the traces, it's very hard to fall into miscalculations that seem plausible but don't actually work out. WinLens3D Basic is a free download for Windows systems. It's certainly not trivial to use, but I've found it well worth the trouble to work through.

Here are a couple of displays from WinLens that demonstrate what I'm talking about with the closeup lens. What I've done here is to configure a 100 mm "main" lens at 1:1, then added an 80 mm closeup lens at various distances from the main lens. The main lens is a triplet from WinLens's library; the closeup lens is modeled as thin.



Notice that when the closeup lens is very close to the subject, the magnification is hardly changed. The perspective is changed a lot however, in this case becoming "inverted" so that parts of the subject farther away will appear larger, not smaller as usual. (See the FAQ: Stopping down a lens combo for more discussion of this effect.)

Adding the closeup lens with 70 mm separation moves the focus point closer in and increases the magnification to 2.533:1. This configuration also turns out to be almost perfectly telecentric -- notice that the centers of the ray cones are parallel to each other as they come off the subject.

Moving the closeup lens even farther toward the main lens further increases the magnification, to 3.158:1 and restores more normal perspective, with the central axes of the ray cones converging from the subject toward a point in back of the lens. I've labeled this "telephoto" perspective because it's the same as what you'd get shooting with a much longer lens from farther back.

I hope this is helpful. The effects I've described here are all "basic optics" by the standards of optics specialists, but for the rest of us they can get pretty mysterious and nonintuitive. Physical experiments combined with ray-trace modeling using some tool like WinLens can do a lot to help keep things straight.

--Rik
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