Macro Flash Calculations

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georgetsmurf
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Macro Flash Calculations

Post by georgetsmurf »

I know that these days most photographers have probably given up worrying about flash calculations as the new equipment works it all out for you however for those experimenting with flash that only have a budget for older equipment this may help.
Example: You have a 90mm macro lens attached to your camera you have it extended by a further 90mm giving you a 1:1 magnification. Your camera is set at 100ASA (ISO 100).

Maths: {1/[(90+90/90)]} all squared = 0.25

Then: 100(ASA) x 0.25 = Effective ASA = 25


This calculation is based on a reflectivity of 18%(standard grey card) and will vary depending on the item you are photographing however it is a reasonable starting point. Also flash calculations tend by my experience to deviate from the physics at close range. Anyway I have found this is better than guess work to get you started.
Cheers George.
I would be very interested to hear other's methods and it may also help those people that are experimenting with older tecnnologies.

AndrewC
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Post by AndrewC »

I think you will find that most people shoot macro with the flash in manual control and use exposure histograms to check exposure. As we typically shoot at constant distances and constant apertures, and more often than not illuminate through diffusers, theoretical calculations of exposure are somewhat abstruse ...
rgds, Andrew

"Is that an accurate dictionary ? Charlie Eppes

georgetsmurf
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Post by georgetsmurf »

AndrewC wrote:I think you will find that most people shoot macro with the flash in manual control and use exposure histograms to check exposure. As we typically shoot at constant distances and constant apertures, and more often than not illuminate through diffusers, theoretical calculations of exposure are somewhat abstruse ...
I guess my example was directed at people photographing in-situ where subject control is limited. I will, when the weather is more condusive do some example shots and post them.

AndrewC
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Post by AndrewC »

georgetsmurf wrote:.. where subject control is limited...
Point taken, but even though your subject control is limited you are taking control of the light by using flash. My working practise is to test exposure for a given setup (lens, diffuser, working distance) on a neutral / inanimate subject prior to approaching a beastie. "Test exposure" for me means evaluating the histogram - I want the maximum info in my saved file so push the histogram to the right with no clipping. I'll then fine tune brightness etc during conversion.
rgds, Andrew

"Is that an accurate dictionary ? Charlie Eppes

georgetsmurf
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Post by georgetsmurf »

Andrew said
My working practise is to test exposure for a given setup (lens, diffuser, working distance) on a neutral / inanimate subject prior to approaching a beastie. "Test exposure" for me means evaluating the histogram - I want the maximum info in my saved file so push the histogram to the right with no clipping. I'll then fine tune brightness etc during conversion.
I am intereted in this approach as a friend of mine tends to do this for all his photography. He is new to the activity and only started taking pictures at the introduction of digital. Me being from the old school of transparencies with minimum latitude find it difficult to go this way. But I guess intuitively, pushing the histogram to the right is attempting to favour the highlights which is what one attempted to do whith Kodachrome. What do you mean by clipping? Unfortunately with trannies we had to get it right first shot and I still tend to work that way. Another one of my interests is car shows-taking some days over 200 images and getting them to a web-site on the same day-no time for re-touching. These habits are difficult to shake. As it is it took me 3 years to start photographing again after the loss of Kodachrome. Stubborn old coot that I am.

AndrewC
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Post by AndrewC »

Expose To The Right (ETTR) basically means that you try to reduce the noise in an image by using the image sensors in a higher resolution part of their response curves. It can be much easier to recover detail from over-exposed (but not blown) highlights rather than underexposed shadows. Having said that, the quality of image sensors nowadays, and the preprocessing done in the camera using firmware like Nikon's Active D-Lighting, means that it is a lot less important than it was on some of the earlier digital cameras. Some people use ETTR as a mantra, others say it is pointless. It needs to be treated as part of your total workflow. For myself, if in doubt about exposure, I tend to overexpose rather than underexpose and then use Photoshop to adjust "exposure" and contrast.

Clipping refers to the histogram you hopefully can see on your camera. If you are overexposing a channel you will see pixels piling up on the right hand side, under exposing they pile up on the left hand side.

This is a good link to understanding histograms:

http://www.cambridgeincolour.com/tutori ... grams1.htm

For me, one of the great things about shooting digital is the speed with which you can evaluate exposure, focus, framing, etc. It encourages me to experiment more and hopefully to learn faster.
rgds, Andrew

"Is that an accurate dictionary ? Charlie Eppes

ChrisLilley
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Re: Macro Flash Calculations

Post by ChrisLilley »

georgetsmurf wrote:I know that these days most photographers have probably given up worrying about flash calculations as the new equipment works it all out for you however for those experimenting with flash that only have a budget for older equipment this may help.
My equipment does not work it out for me (neither flash settings nor exposure) because I choose to use older, manual focus lenses most of the time and my camera (Nikon D90) does not even know a lens is connected if there is no electronic coupling.

So I shoot in manual mode most of the time.

I also use flashes (Vivitar 285HV) which have manual power settings.
georgetsmurf wrote: I would be very interested to hear other's methods and it may also help those people that are experimenting with older tecnnologies.
I don't use calculations to judge exposure. I iterate from a best guess, examining the histograms and adjusting. Final corrections are done during raw developing.

This is a big difference with digital compared to film. Feedback is instant, and the cost of taking trial shots is zero.

georgetsmurf
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Re: Macro Flash Calculations

Post by georgetsmurf »

ChrisLilley wrote
My equipment does not work it out for me (neither flash settings nor exposure) because I choose to use older, manual focus lenses most of the time and my camera (Nikon D90) does not even know a lens is connected if there is no electronic coupling. So I shoot in manual mode most of the time.
I feel the same as you do, in fact from trying many medium priced digital lens aquired then re-sold on Ebay I have found it is difficult to find lenses as sharp as my old Nikon or SP Tamrons. For my car photography I did manage after many attempts to find a lens that does the job-a Nikon DX 17-50mm f2.8 all the way through, even with this one I had to buy two to find one that was good enough. This I can use on Aperture priority successfully. For my macro work my best lens is a SP Tamron 90mm f2.5 that I purchased in the 1980's that I use fully manual on my D300s. Or on older Nikon film bodies that i still have. In particular FM2's. I still shoot some film (Velvia Pro). Surprisingly, I get better colour saturation and resolution with film for extra special car shots. I have a Nikon 4000dpi scanner (film and slide dedicated) to turn them into digital for distribution etc.

Harold Gough
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Post by Harold Gough »

Just one comment. For macro work the change in light intensity varies in +/_ inverse proportion to the distance of the flashgun from the subject, not its square, as in normal photography.

Harold
My images are a medium for sharing some of my experiences: they are not me.

georgetsmurf
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Post by georgetsmurf »

Harold Gough wrote:Just one comment. For macro work the change in light intensity varies in +/_ inverse proportion to the distance of the flashgun from the subject, not its square, as in normal photography.

Harold
Interesting. If that is the case at what point does "normal photography" become macro photography? I would have thought light intensity is always a inverse square function of distance from the source with the exception maybe of a concentrated parellel or controlled-focus beam eg laser. :?:

Harold Gough
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Post by Harold Gough »

georgetsmurf wrote:I would have thought light intensity is always a inverse square function of distance from the source with the exception maybe of a concentrated parellel or controlled-focus beam eg laser. :?:
It applies to a point source, not a "wall of light" such as the sun in the sky or a flashgun at close quarters. Pop-up flash guns may give an intermediate situation but I have no experience.

This is a table I worked out for my own use many years ago:


MANUAL FLASH GUN DISTANCES (INCHES) FOR 50ASA# WITH SPECIFIED FLASHGUNS##

Magn 2* 1.5* 1* 2/3* 1/2 1/3 1/4 1/5-1/10 1/10-1/20
f32 4 5 8 12 14 15 16 17-19 19-20
f22 5 7 12 18 21 22 25 25-27 27-29
f16 7 10 17 25 28 3 33 35-38 38-40
f11 10 14 25 36 42 45 50 50-55 56-58
f8 14 20 34 50 56 62 66 70-76 77-81
f5.6 20 28 50 72 84 90 100 100-110 112-116
f4 28 40 68 100 112 124 132 140-152 153-162

#Increase distance by ca 5% for 64ASA
## Main one GN 60ft with 25ASA
* Allow for any extension e.g. 2x converter

While the table format is retained in the draft it is lost in the preview and submission. I think you can get the idea.

Harold
My images are a medium for sharing some of my experiences: they are not me.

georgetsmurf
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Post by georgetsmurf »

Harold Gough wrote:
georgetsmurf wrote:I would have thought light intensity is always a inverse square function of distance from the source with the exception maybe of a concentrated parellel or controlled-focus beam eg laser. :?:
It applies to a point source, not a "wall of light" such as the sun in the sky or a flashgun at close quarters.


Yes, I realise that-usually from an artificial light perspective the light source for inverse square law definiton is a bare incandescent light bulb.

It would be interesting to know what calculations/algorithms are used for TTL metering with a dedicated ring flashes.
Thanks for these data anyway-I think I will find them useful. Cheers George.

AndrewC
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Post by AndrewC »

georgetsmurf wrote:...
It would be interesting to know what calculations/algorithms are used for TTL metering with a dedicated ring flashes.
....
They measure photons rather than juggle electrons ! The reason why some cameras can't TTL meter macro flash is because they either can't measure the amount of reflected light for such a short exposure or the quench circuit can't dump the excess charge fast enough.
rgds, Andrew

"Is that an accurate dictionary ? Charlie Eppes

georgetsmurf
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Post by georgetsmurf »

Georgetsmurf wrote: It would be interesting to know what calculations/algorithms are used for TTL metering with a dedicated ring flashes.
....
Andrew wrote: They measure photons rather than juggle electrons ! The reason why some cameras can't TTL meter macro flash is because they either can't measure the amount of reflected light for such a short exposure or the quench circuit can't dump the excess charge fast enough.
Andrew, in that case, what would be a typical flash duration for a ring flash at such close distances? I recall one of my older flash units stating up to (or down to, whichever way you look at it ) around a 1/10,000 Second. Maybe that is why it seems to become inconsistent when too close to the subject. Or maybe it is like the shutter speed tolerances of most cameras. When I had my mechanical camera repaire business I found that only Leica and Haselblad had consistently accurate shutters, that is whithin 30% while Nikon and Pentax were good for about 50% except when you reached the say 1/4000 second on the Nikon FM2's but that may have been my equipment that I made myself based on a electronics magazine design but using closer tolerance resistors:?

ChrisR
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Post by ChrisR »

Maybe that is why it seems to become inconsistent when too close to the subject. Or maybe it is like the shutter speed tolerances
Remember the shutter just has to be 100% open when the flash goes off so its duration doesn't matter.
Even studio flash on quarter power varies its output a bit from frame to frame. Camera-mountable ones seem to be worse, there are always a few frames in a stack which jump out as different :evil:
Last edited by ChrisR on Fri Jan 07, 2011 3:45 pm, edited 1 time in total.

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