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02-16-2014, 10:46 AM   #31
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Do not know if the entire frog needs to be in focus, would be better for the OP, maybe going to anaesthetise the frogs?

---------- Post added 02-17-14 at 04:51 AM ----------

QuoteOriginally posted by fretlessdavis Quote
Another thing to consider, is that since you're on APS-C, close-up filters work quite well. With a +3 Hoya Close-up on an SMC-M 50mm f/1.7, this is what I got straight out of camera:
Good photo by way. Interesting that the working distance stays much the same, how do filters effect depth of field, or they do not?

02-16-2014, 10:57 AM   #32
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QuoteOriginally posted by gmans Quote
Do not know if the entire frog needs to be in focus, would be better for the OP, maybe going to anaesthetise the frogs?
Haha. That would make things a lot easier!

A note on the DOF calculator. I believe it uses an 8x10 at 300 dpi as standard reference for DOF, and selects a COC to reflect that.

I didn't know anything about COC (circle of confusion) and enlargement until I got into Large format. For example, a shot on LF printed at 8x10 will have a lot more apparent DOF than one printed at 20x24. You have to match your output size to your COC to get accurate DOF calculations.

If the intended output is for we viewing, at 800 pixels wide, the COC is a LOT greater, so the final images will have a lot more apparent DOF than they would if they were printed out. Same thing with stopping down affecting sharpness-- for 800 pixel wide web prints, you can stop down a lot further and retain 'sharpness', since only 16 lp/mm of resolution will be displayed. So, even with APS-C, at that output size, you could stop down to f/22-f/32 and still get good results. Paired with the larger COC of that output, apparent DOF will be usable.

For example

---------- Post added 02-16-14 at 11:00 AM ----------

QuoteOriginally posted by gmans Quote
Do not know if the entire frog needs to be in focus, would be better for the OP, maybe going to anaesthetise the frogs?

---------- Post added 02-17-14 at 04:51 AM ----------



Good photo by way. Interesting that the working distance stays much the same, how do filters effect depth of field, or they do not?
EDIT: the filters don't keep the same working distance, but they seem to elongate the focal length a bit. For example, getting 1:2 with a 50mm lens needs 100mm, but with the filters getting there needs about 128mm. Maybe just distortion or optical imperfections, though... I am sorry and didn't mean that workign distance was the same with just popping a filter on.

Thanks!

DOF is pretty close to what it would be at that magnification range with any other lens, I guess.

At a fixed magnification ratio, the DOF will be pretty consistent between all lenses, actually. Try it out with the DOF calculator. a 50mm lens @100mm (subject distance) will give 1:2, and a 100mm lens will get you there @200mm (subject distance). They'll give the same DOF.

Last edited by fretlessdavis; 02-16-2014 at 11:04 AM.
02-16-2014, 11:08 AM   #33
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QuoteOriginally posted by fretlessdavis Quote
f the intended output is for we viewing, at 800 pixels wide, the COC is a LOT greater, so the final images will have a lot more apparent DOF than they would if they were printed out. Same thing with stopping down affecting sharpness-- for 800 pixel wide web prints, you can stop down a lot further and retain 'sharpness', since only 16 lp/mm of resolution will be displayed. So, even with APS-C, at that output size, you could stop down to f/22-f/32 and still get good results. Paired with the larger COC of that output, apparent DOF will be usable.
Will take a time out to digest this. Cheers
02-16-2014, 11:09 AM   #34
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Thanks for all the advice! Some clarifications:

Yes, the frogs will be anaesthetized. That's why I mentioned that they will stay still for me. Also (I mentioned this in the post too, but it was long so no worries if you missed it) I'm not going to be changing the working distance to match the frog size. Rather than having them fill the frame regardless of size, I want the largest frogs (approx 75mm) to fill the frame, and the smaller frogs to fill a proportionately smaller amount. That is to say, I'll be shooting at the same zoom and working distance regardless of frog size this is so that I can easily compare relative sizes across images. The camera will be on a tripod with a swing arm that allows it to be held facing downward at the required distance above the ground, and triggered remotely.

Depth of field is indeed important. I'm not expecting the frogs to be razor-sharp all the way down, but I need everything to be at least sharp enough that I can take measurements. The aesthetic quality of the photos is not particularly important, but they need to be distortion-free for accurate measuring and I need to be able to see everything at least relatively clearly.

Does that help narrow down the recommendations? If there is anything else that I can help clarify, please let me know.

02-16-2014, 11:11 AM   #35
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Oh, and fretlessdavis: I'd be happy to buy those filters off you and try them out (like you said, I don't have much to lose at $5) but I can't seem to find the thread in the marketplace where you're offering them. If you could direct me to it, that would be awesome of you.
02-16-2014, 11:20 AM   #36
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Try it this way:

Take a good shot of something where the DOF falls away gracefully (like a book on a table or something).

Zoom out to about what would be 800 pixels wide (since the pixel size of your monitor is fixed, this is a good representation of what it would look scaled to output)

Pick the place where you think the DOF falls off to being unacceptably sharp. Start zooming in on that spot. The more you zoom in, the more you will notice the lack of sharpness in that area. Now, after zooming for a bit, pick the new place where DOF falls of to being unacceptably sharp. Repeat a couple times until you're looking at it scaled to how you would critically look at an 8x10. Mark that spot, and zoom out to 800 pixels again. It's amazing how much DOF you'll 'gain'.

I've done quite a bit of product photography, and as soon as I learned this, my life got a lot easier. f/22 and the 1.5 MP setting on my K100d became perfectly usable for eBay sized photos. Taken at 6MP and looked at critically, they were pretty soft and lacking some DOF. However, they looked perfectly sharp all across the board when viewed at the 200-500 Pixel wide sizes when you look through things on eBay. At f/22 and typically a 1:8 ratio or more, I rarely even had to refocus between shots!

---------- Post added 02-16-14 at 11:28 AM ----------

QuoteOriginally posted by Frog_Botherer Quote
Oh, and fretlessdavis: I'd be happy to buy those filters off you and try them out (like you said, I don't have much to lose at $5) but I can't seem to find the thread in the marketplace where you're offering them. If you could direct me to it, that would be awesome of you.
Whoops! I forgot to repost the ad. I withdrew it to give away an ME Super for free.

https://www.pentaxforums.com/forums/24-photographic-equipment-sale/251856-sal...p-filters.html

I'll PM you my info.

---------- Post added 02-16-14 at 11:38 AM ----------

QuoteOriginally posted by Frog_Botherer Quote
Thanks for all the advice! Some clarifications:

Yes, the frogs will be anaesthetized. That's why I mentioned that they will stay still for me. Also (I mentioned this in the post too, but it was long so no worries if you missed it) I'm not going to be changing the working distance to match the frog size. Rather than having them fill the frame regardless of size, I want the largest frogs (approx 75mm) to fill the frame, and the smaller frogs to fill a proportionately smaller amount. That is to say, I'll be shooting at the same zoom and working distance regardless of frog size this is so that I can easily compare relative sizes across images. The camera will be on a tripod with a swing arm that allows it to be held facing downward at the required distance above the ground, and triggered remotely.

Depth of field is indeed important. I'm not expecting the frogs to be razor-sharp all the way down, but I need everything to be at least sharp enough that I can take measurements. The aesthetic quality of the photos is not particularly important, but they need to be distortion-free for accurate measuring and I need to be able to see everything at least relatively clearly.

Does that help narrow down the recommendations? If there is anything else that I can help clarify, please let me know.
Given your requirements, I think stopping down won't be a big issue. If the primary purpose is for measuring, you can squeeze a little more DOF out of shots, since sharpness doesn't need to be critical.

If you have a ruler in the edges of your frame, I actually think something like the 50mm or 100mm f/4 Macro Takumar/SMC-M macro, or even an enlarger lens on bellows, and fix focus on the ruler. A flat-field lens like the ones there will have no distortion along the edges. Ken Rockwell did a pretty in depth test of the 100mm macro, and there was *no measurable distortion* at ANY distance. Given that you're shooting at the same magnification ratio, though, you could easily correct for distortion from any lens, and apply the same correction to every image you take, letting you use any lens, tube, filter, bellow, etc combination you want.
02-16-2014, 12:56 PM   #37
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QuoteOriginally posted by Frog_Botherer Quote
Does that help narrow down the recommendations? If there is anything else that I can help clarify, please let me know.
What do you expect will be the maximum depth that needs to be in reasonable focus?
02-17-2014, 09:40 AM   #38
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QuoteOriginally posted by Frog_Botherer Quote
(there will be a ruler in the frame for measurement calibration)
I missed that part of the OP when I replied earlier. I thought you wanted the same magnification for all frogs, but now it's clearer that you want variable magnification to get the frog to fill the frame. It looks like the Raynox DCR-150 is a good match with your existing DA 18-135 lens. You'll be able to cover magnification rates from approx. 0.1x all the way to 1.0x. Frogs 25-75mm long will fill the sensor as you zoom.

https://www.pentaxforums.com/forums/10-pentax-slr-lens-discussion/130066-how-...tion-rate.html provides more details about determining Raynox magnification rates..

02-17-2014, 10:26 AM   #39
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I think a 50mm lens would be too close and too dark. Assuming you don't spook the frogs, you'd have to make sure you weren't in the way of the light (or spring for a ring flash). The 100mm lens would be better for the longer working distance..
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