Calculating focus stack increment by magnification
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Calculating focus stack increment by magnification
I just shot a subject that was 9mm thick by changing the bellows draw .1mm per frame. Initial bellows draw was approximately 180mm. I was expecting it to take about a 100 images to complete the stack, but it took 388. The change in bellows draw was +39mm. With a 4/3rds sensor, what increment should the frames have been shot for complete coverage (allowing for small overlap as well)?
Lens was El-Nikkor 50mm 2.8 at f5.6.
p.s. Photoshop is still busy building the previous stack, so no image to show
Lens was El-Nikkor 50mm 2.8 at f5.6.
p.s. Photoshop is still busy building the previous stack, so no image to show
- rjlittlefield
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Ed,
Calculations are not very reliable for this sort of problem. It's much better to run a short test stack with fine spacing, then examine the results to see how much bigger you can get away with.
Having said that, let me calculate...
With a 180 mm draw, call it 200 mm sensor to lens, you're apparently working at about 3X. 4/3rds sensor is 18 mm wide, so your subject is about 6 mm wide. But you say it's 9 mm deep. If I'm correct so far, then the situation is that you're trying to shoot a high magnification stack that has more depth than width. That pops up all sorts of red flags because the higher the magnification, the harder it is to handle large ratios of depth:width.
But already I see a problem with calculating from what's given. When I plug in the 50 mm focal length, 200 mm initial lens-to-sensor, and 39 mm additional draw, I get a depth of about 3.5 mm, not 9 mm. This is from the thin lens equation, 1/f = 1/o + 1/i, plugging in f=50 and i=200 vs i=219. Even using i=180 and i=219 only gives 4.5 mm depth. So I'm not sure if I've blown the calculation or if there's something wrong with the description of the setup.
At any rate, proceeding with the calculation...
For subjects of width about 6 mm, and an f/5.6 setting on the lens, experience leads me to believe that you're going to need something like 0.05mm focus step. (Beware: focus step is different from bellows draw step!) See http://www.photomacrography.net/forum/v ... php?t=6221 for NikonUser's experiments at f/5.6 with the greenbottle fly.
So, if you're serious about the full 9 mm, I'm thinking that you really will need something like 180 steps (9/0.05 = 180) to have no visible banding.
One other issue crops up with your particular setup. Focusing by changing the bellows draw has the nice feature of keeping the entrance pupil perfectly fixed. But it has the very un-nice feature that focus step is not a fixed multiple of bellows step. Going back to the thin-lens equation, we can calculate that changing bellows draw from 180mm to 180+0.05mm produces a focus step of 0.0074mm, but changing the draw from 219mm to 219+0.05mm produces a focus step of only 0.0044mm.
In other words, the bellows step at 219mm changes focus by only 60% as much as the step at 180mm did.
The message here is that if you determine focus step experimentally -- which I strongly recommend -- then you should do that experiment at the minimum bellows draw that you plan to use. That way you'll end up with a value that is too conservative at maximum draw, but at least you'll still get a good result. If you do the experiment at maximum bellows draw, you'll end up finding a value that is too big at minimum draw, and you'll end up with OOF bands as a result.
I hope this is helpful. The task you're trying to do is quite challenging.
--Rik
Calculations are not very reliable for this sort of problem. It's much better to run a short test stack with fine spacing, then examine the results to see how much bigger you can get away with.
Having said that, let me calculate...
With a 180 mm draw, call it 200 mm sensor to lens, you're apparently working at about 3X. 4/3rds sensor is 18 mm wide, so your subject is about 6 mm wide. But you say it's 9 mm deep. If I'm correct so far, then the situation is that you're trying to shoot a high magnification stack that has more depth than width. That pops up all sorts of red flags because the higher the magnification, the harder it is to handle large ratios of depth:width.
But already I see a problem with calculating from what's given. When I plug in the 50 mm focal length, 200 mm initial lens-to-sensor, and 39 mm additional draw, I get a depth of about 3.5 mm, not 9 mm. This is from the thin lens equation, 1/f = 1/o + 1/i, plugging in f=50 and i=200 vs i=219. Even using i=180 and i=219 only gives 4.5 mm depth. So I'm not sure if I've blown the calculation or if there's something wrong with the description of the setup.
At any rate, proceeding with the calculation...
For subjects of width about 6 mm, and an f/5.6 setting on the lens, experience leads me to believe that you're going to need something like 0.05mm focus step. (Beware: focus step is different from bellows draw step!) See http://www.photomacrography.net/forum/v ... php?t=6221 for NikonUser's experiments at f/5.6 with the greenbottle fly.
So, if you're serious about the full 9 mm, I'm thinking that you really will need something like 180 steps (9/0.05 = 180) to have no visible banding.
One other issue crops up with your particular setup. Focusing by changing the bellows draw has the nice feature of keeping the entrance pupil perfectly fixed. But it has the very un-nice feature that focus step is not a fixed multiple of bellows step. Going back to the thin-lens equation, we can calculate that changing bellows draw from 180mm to 180+0.05mm produces a focus step of 0.0074mm, but changing the draw from 219mm to 219+0.05mm produces a focus step of only 0.0044mm.
In other words, the bellows step at 219mm changes focus by only 60% as much as the step at 180mm did.
The message here is that if you determine focus step experimentally -- which I strongly recommend -- then you should do that experiment at the minimum bellows draw that you plan to use. That way you'll end up with a value that is too conservative at maximum draw, but at least you'll still get a good result. If you do the experiment at maximum bellows draw, you'll end up finding a value that is too big at minimum draw, and you'll end up with OOF bands as a result.
I hope this is helpful. The task you're trying to do is quite challenging.
--Rik
Holy cow, 388 images deep stack, that is really extremely deep!
I've shot up to around 200 images deep and you need plenty of memory and scratch disc space to handle that. I seem to remember reading here somewhere that PS was more resource intensive than the dedicated stacking software in which case I'd suggest you give the stack a go in Tufuse or CZP if PS ends up exploding!
I've shot up to around 200 images deep and you need plenty of memory and scratch disc space to handle that. I seem to remember reading here somewhere that PS was more resource intensive than the dedicated stacking software in which case I'd suggest you give the stack a go in Tufuse or CZP if PS ends up exploding!
Rik: Thanks, that helps. It seems that changing the bellows draw can give finer control over the focus change at the cost of being variable between each change. I'll need to redo my tables in Excel now
Laurie: Once you've shot 50 or 60 frames, it's pretty hard to stop I did just get a 133x 8gb memory card and the save time for individual images is about 2 seconds now. It was 15 seconds with my old card. It's a lot less painful now to shoot lots of images.
Laurie: Once you've shot 50 or 60 frames, it's pretty hard to stop I did just get a 133x 8gb memory card and the save time for individual images is about 2 seconds now. It was 15 seconds with my old card. It's a lot less painful now to shoot lots of images.
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I was just passing through, being a film user and thus not being a stacking practitioner.
I'm curious. If I understand the question, this stack would have a continuously varied magnification from one end to the other. Or have I missed something?
Wandering off on the internet, in a daze from these technicalities, I came across this, which I suspect has been posted more than one previously. I leave others to judge its relevance to this thread:
http://www.apug.org/forums/forum44/5567 ... nsion.html
I'm curious. If I understand the question, this stack would have a continuously varied magnification from one end to the other. Or have I missed something?
Wandering off on the internet, in a daze from these technicalities, I came across this, which I suspect has been posted more than one previously. I leave others to judge its relevance to this thread:
http://www.apug.org/forums/forum44/5567 ... nsion.html
My images are a medium for sharing some of my experiences: they are not me.
- rjlittlefield
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You're correct -- the magnification varies from one end to the other.Harold Gough wrote:I'm curious. If I understand the question, this stack would have a continuously varied magnification from one end to the other. Or have I missed something?
Stacking software handles this by figuring out what the necessary re-scaling is for each image, to make the various images line up correctly so you don't get duplicated details, smearing, etc.
It is not so obvious, but magnification changes from one end to the other in almost all stacks, regardless of how you do the focusing. The only real exception is if you use telecentric optics.
--Rik
I have made over 27,000 exposures in less than 2 years. The shutter on the D2X is given a conservative estimate of 150,00 cycles - so mine is still a baby.
NU.
student of entomology
Quote – Holmes on ‘Entomology’
” I suppose you are an entomologist ? “
” Not quite so ambitious as that, sir. I should like to put my eyes on the individual entitled to that name.
No man can be truly called an entomologist,
sir; the subject is too vast for any single human intelligence to grasp.”
Oliver Wendell Holmes, Sr
The Poet at the Breakfast Table.
Nikon camera, lenses and objectives
Olympus microscope and objectives
student of entomology
Quote – Holmes on ‘Entomology’
” I suppose you are an entomologist ? “
” Not quite so ambitious as that, sir. I should like to put my eyes on the individual entitled to that name.
No man can be truly called an entomologist,
sir; the subject is too vast for any single human intelligence to grasp.”
Oliver Wendell Holmes, Sr
The Poet at the Breakfast Table.
Nikon camera, lenses and objectives
Olympus microscope and objectives
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I do a lot of panoramics and quite a few stereo pairs and it certainly eats up the film. Mind you, I get a lot more final images for my money. Investing in hundreds of films of high quality, in date, but discontinued film stock at about 10% of RRP made it affordable, at the cost of half a freezer's capacity.
Harold
Harold
My images are a medium for sharing some of my experiences: they are not me.
- rjlittlefield
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