Depth of field with Close-Up lens

[Graystar]

Does anyone know how to calculate the various DOF distances (near, far, sub, etc.) when a close-up lens has been attached to a compact camera?

[McQ]

It can become a little difficult if you account for all effects. Unless you need something super precise, I would just use the standard depth of field equations (the ones which are approximations for when the magnification is less than 1:1). Part of the problem with calculating this for a macro lens is because 2nd order lens effects begin to come into play. This means that you will need to know a lot more about the inner workings of your lens than just its focal length and your sensor size. The biggest 2nd order effect to consider is the pupil magnification, which acts to increase the DoF beyond what the standard DoF equations would predict. Do a google search on the "pupil magnification" and you will see how things can get messier...

[Graystar]

Thanks. I searched and read up on pupil magnification...yeah it’s complicated.

Seems like every DOF calculator I’ve tried...even a couple of macro ones...comes up with the wrong answer.

The reason I was asking is because I seem to get less DOF when using the close-up lens as opposed to the camera's built in macro function. This doesn’t seem to make sense, as I need to be about 8-9 inches away from the subject when using the lens, and only 1-2 inches away for similar framing when using the camera macro function. I was hoping to find out the exact difference in DOF.

[Dave Humphries]

Hi guys,

I was wondering whether you'd be quicker to just take a series of test photographs with the lens and lens+adaptor of say, a ruler (end on), or something home made that might be more fit for purpose (i.e. easier to read) - just an idea.

I was thinking you could then tabulate the results in a spreadsheet and extrapolate the readings into a graph for reference when you need it. Just don't ask me how 'cos that's beyond my skills, sorry.

Good luck, Dave

[Graystar]

Yeah I got tons of comparison pics. I was hoping to get a more scientific perspective on the mechanics.

[Dave Humphries]

Ah, forgive my obvious suggestion; so obvious you've already been there, done that and got the T-shirt.

Let me give it some thought, I can now see these shots are exceedingly difficult to interpret.
In fact, in future, I might just try my ideas out myself first (before making daft suggestions).

Apologies

[Dave Humphries]

OK, I've had a thunk about the problem with "ruler" shots and this is what I came up with. I have even tried it myself - but very crudely; it is hand drawn, hand held, 400ISO, etc.

A plan of what I drew;
DOF-checker-plan-view-1280.jpg

The idea is to get the 'fan' of lines to be a constant spacial frequency horizontally, to allow measurement of sharpness at different distances from the lens, merely by moving up and down the image vertically. A crude scale (at 5mm increments) is provided to then ascertain the distance considered "sharp enough" to be judged in focus.

The camera is angled and moved around until the lines ALL appear vertical and parallel in the viewfinder, then pictures are taken at various f-numbers. With my bridge camera's 10x zoom lens at 28mm equiv, (6.2mm actual) focal length on "super macro" setting, there is significant barrel distortion evident, but you get the idea.

At f2.8;
DOF-checker-at-f2-8-1600.jpg

At f4;
DOF-checker-at-f4-0-1600.jpg

At f7.1 (I was aiming for f8!);
DOF-checker-at-f7-1-1600.jpg

I accept it is a lot of trouble to go to, and a mathematical solution is preferable, but I was intrigued by the difficulty of interpretting the "ruler" shots and this may help.

If anyone thinks the idea is worth following up, they might like to try this scaled better (more 'fan' lines and better measure scale), printed from a PC, etc.

Be my guest.

Graystar, I don't think this helps you much, but it was an interesting 1/2 hour diversion for me, at least.

Regards, Dave

[Graystar]

I’ve done more testing and found that the DOF with a close-up lens is extremely shallow.

I took more pictures of the ruler, however I positioned the ruler as projecting straight out in line with the lens, and slightly below the center of the lens. This made it very difficult to read the ruler, but I realized that the angle created by having the camera up over the ruler was causing the perceived DOF to be way off.

With the ruler now showing accurate distances from the camera, I could see that the depth of field is extremely shallow with the close-up lens, and seems consistent with the calculations from DOF calculators. With my combination of A710 and 250D lens, at 10 inches, f/8.0, and max zoom I only have about 0.15” of total DOF. That’s not much at all.

Fortunately, using the CHDK hack I can set the aperture to f/14.25, which gives me another tenth of an inch.