Perplexed with an extension tube

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Kevin Childress
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Perplexed with an extension tube

Post by Kevin Childress »

Looking for help to understand a problem with image quality. I'm shooting a D810 with Nikkor 105mm f/2.8 micro. The extension tube is a variable tube (55 - 71mm) from Savage and seems like a very well built piece of equipment. I used the tube at 55mm and noticed right away the extension tube dramatically increased the depth of field, and I do mean dramatically. This seems backwards to me but the DoF was increased so much that I captured a focus stack with only 30 frames where I would typically need 120+ frames to cover the same linear distance. I was really beginning to think my life had just gotten a lot easier! But wait ...

When I imported the images into Lightroom I was shocked at what I saw. I had 30 absolutely fuzzy images. I can't understand it but there isn't one sharp pixel in any of the 30 images. Thinking something might be wrong with the camera I removed the extension tube and re-shot the entire project using my 105mm f/2.8 alone at 1:1 and the results were perfectly normal.

Attached are a couple comparison shots. The first comparison shows an image (on left) shot with the extension tube and an image (on right) shot with the macro lens alone, both at f/16. I looked through all 30 images shot with the extension tube and this is the sharpest image I can find of that area of my subject. The second image: I was so perplexed I set up again for one more test. For my macro projects I use a 24-inch HDMI monitor connected to the camera for analyzing critical focus and it really works well. I chose an area of the subject around the mouth and bottom of an eye and shot two images - one with the extension tube at 71mm and the lens at 1:1 and another image without the extension tube and at 1:1, both at f/16. Same results!

Third image: I began to wonder if moving the lens diaphragm away from the image sensor had somehow changed the effective f/stop so I ran one more test. I shot frames at f/8, f/5.6, and f/4.8 and I can't tell any difference at all between them. Also attached is a comparison of f/16 and f/4.8.

Any ideas?



Image
Image
Image

rjlittlefield
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Post by rjlittlefield »

Have you checked the exposure times on those images?

I'm thinking that somehow the extension tube is causing your lens to be stopped clear down at all times. That would explain the lack of difference in appearance, as well as the increased DOF and correspondingly increased diffraction blur causing the lack of sharpness.

--Rik

Kevin Childress
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Joined: Fri Aug 14, 2015 2:19 pm
Location: Lowell, North Carolina
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Post by Kevin Childress »

Rik,

I'm not sure what you mean about checking the exposure time. I mean, I know all the images shot with the extension tube were at 1/250th, but not sure what else there is to know about it.

Your idea about the lens being stopped down all the way, all the time, is as good of an explanation as I can think of. I think I'll contact savage and pose that scenario to them ...

Thanks for the reply!

Kevin

rjlittlefield
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Post by rjlittlefield »

Kevin Childress wrote:I know all the images shot with the extension tube were at 1/250th, but not sure what else there is to know about it.
That's enough, since equal exposure brightness at equal times confirms that the actual aperture was not changing, despite what the annotation says.

I'll be interested to hear what Savage has to say.

--Rik

Pau
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Post by Pau »

Kevin Childress wrote:I know all the images shot with the extension tube were at 1/250th, but not sure what else there is to know about it.
1/250 :smt017 ....Did you use flash in auto mode?
In that case the exposure is made by the flash and Rik's explanation would still apply. I can't find any alternative.
Pau

Chris S.
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Post by Chris S. »

Kevin,

I agree that it sounds as if all shots made with the extension tube are occurring with the lens fully stopped down. If so, this is an something you can check through direct observation. Try this:

1) Put just macro lens on the body, set it for a middle aperture such as f/8, set shutter speed to "bulb," and look into the front of the lens. Depress shutter button. You should see the iris go from fully open to partially stopped down. When you release the shutter button, the iris should pop back to fully open. Good so far?

2) Now put the extension tube on the camera, and the lens on the extension tube. Repeat the exercise above. Ideally, what you see should echo what you saw in step one. But from what you're experiencing, there is a good likelihood that you'll see something different. What do you see?

Looking at the reviews at B&H, some users report issues with fit at either the camera or lens end of this extension tube. Your issue could fall into this category.

--Chris S.

--edited typo
Last edited by Chris S. on Mon May 08, 2017 10:07 pm, edited 1 time in total.

Kevin Childress
Posts: 24
Joined: Fri Aug 14, 2015 2:19 pm
Location: Lowell, North Carolina
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Post by Kevin Childress »

Rik, Pau, and Chris,

Thanks to you all for the help. I felt like an idiot as soon as I read the first replies here. I should have realized the problem when I shot those other f/stops and didn't compensate for exposure. So yeah, the aperture is staying stopped down - all the way down - all the time.

I read the B&H reviews before buying this tube and saw where others experienced a tight fit to the lens. My tube is also tight to the lens but it didn't feel like there was danger of damaging the lens.

I'll still call Savage tomorrow just to see what they say about it, but I'll also plan on just exchanging the tube with B&H. Granted I had this problem, I really like the design otherwise. Hopefully I'll get a good copy next time. I'll let you guys know what Savage has to say.

Thanks again!

Kevin

Kevin Childress
Posts: 24
Joined: Fri Aug 14, 2015 2:19 pm
Location: Lowell, North Carolina
Contact:

Post by Kevin Childress »

Closing the loop (for now at least) ...

I spoke to Savage about the tube keeping the aperture stopped down all the time. They commented on having seen a few issues where the tube wouldn't "click" into place on the lens but they didn't seem to want to discuss reports of an overly tight fit to the lens and didn't confirm nor deny having seen my particular problem.

Savage was willing to give me an RMA for returning the tube directly to them whereupon they would then "determine" if they would exchange it for me. Frankly I couldn't get a guarantee that I would receive an exchange with no questions asked. Okayyyy ...

Avoiding any further frustration I simply returned the tube to B&H for an exchange. I received the replacement tube today and all seems to be right with the world. It definitely doesn't fit as tightly to the lens as the previous tube and I definitely have a full aperture range.

Now back to productive testing ...

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