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 ----------
Originally posted by Frog_Botherer 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 ----------
Originally posted by Frog_Botherer 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.