I'm long used to a 50mm SLR film camera and am shopping for my first digital camera. I like a lot that I read about the Olympus C-2500L feature-wise, but the limited f-stops really bug me: How will I be able to get decent depth of field with a maximum f/5.6? On the other hand, I am not used to zoom lenses. My experience is only with a 50mm lens: with it, I know that I need f/22 for some landscape work I sometimes do. My question is: at the short focal length of 28mm in the Olympus C-2500L, will the f/5.6 maximum satisfy me (as far as depth of field goes) if I am used to using f/22 with a 50mm lens to get my maximum depth of field?

-- Linda Hayward (llayward@psn.net), December 09, 1999

Answers

Linda, the shorter focal length will give greater depth of field at any given f-stop value. The other issue in your favor is the much smaller format. The small size of the CCD causes depth of field to be much greater than you would get with the same f-stop in 35mm. In fact, this is a frequent complaint in the digital world. It is very difficult to get a shallow depth of field when you want one. I think you will probably get all the depth of field you want.

-- Steve (milwaukeechrome@aol.com), December 09, 1999.

According to this, you are mistaken about the specs of this camera: it zooms from 9.2 to 28mm, so the wide- angle is 9.2mm (same angle as 36mm lens on 35mm format film).

The aperture required for the same depth of field will be about four times. So the DoF at 9.2, f/5.6 will be about the same as a 36mm lens at f/22 on 35mm film.

-- Alan Gibson (Alan.Gibson@technologist.com), December 10, 1999.

Cool!
I learned something today. I'd never thought about it but the smaller CCD does effect Depth of Field because it takes less glass to do f5.6 on the small CCD then on a 35mm frame. I just never put 2 + 2 together to come to this conclusion.
Thanks gang!

Des

-- Dan Desjardins (dan.desjardins@avstarnews.com), December 10, 1999.

Thanks to all. This makes things a lot clearer. (Yes, I misquoted the minimum focal length equivalent to 35mm film...it is 36mm as Alan says. Sorry about the confusion.)

-- Linda Hayward (llayward@psn.net), December 10, 1999.

At the risk of making a simple explanation more complex, I should point out the the small size of the CCD decreases the depth of field. But the correspondingly shorter focal length compensates for this, and has the same effect again, so the overall result is an increase in depth of field.

Digicams get their high depth of field from short focal lengths, not from the small CCD size.

-- Alan Gibson (Alan.Gibson@technologist.com), December 10, 1999.

A little knowledge is a dangerous thing - so don't shoot! :) Isn't the f number for a lens related to the amount of light (or potential light capacity) for that lens? So isn't a 50 mm at f5.6 going to give the same image and depth of field depending on lens to filmplane/sensor distance no matter the size of the film plane or sensor or camera it's used on? The smaller image on digital will give a telephoto effect, just as a 50 mm on a medium format camera is a wide angle? So on a given lens isn't the DOF at f5.6 still shallower than at f22. So with the limited ranges on the digitals aren't we giving up the DOF control? I know that most of the concern is at getting faster capabilities for sports/action, not "deeper." Or am I just really confused and it's time to revisit Photo 101.

-- Craig Gillette (cgillette@thegrid.net), December 10, 1999.

I don't want to beat a dead horse, but decreasing the format increases depth of field. A very well written article by Bruce Barrett is available at http://www.best.com/~bbarrett/large_format/DOF_text.html

He very clearly lays out all the calculations and concludes that DOF decreases as the size of the format increases. This is why photographers shooting large format cameras use such small apertures to get the needed DOF for landscapes. Conversely, our small CCD's achieve great depth of field even at realitively large apertures. I encourage everyone to have a look at this site. It explains a lot.

-- Steve (milwaukeechrome@aol.com), December 11, 1999.

Craig: don't forget that the negative (or bit-image) gets enlarged. Sure, a 50mm at f/5.6 givs the same DoF on the film/CCD whatever the size of the film, but the degree of enlargement is greater for small sensors. Therefore the acceptable circle of confusion will be smaller.

Steve: Bruce states the situation correctly, but he doesn't say what you think he says. Read his conclusion carefully. He correctly says that the decreased DoF in LF cameras is due to the increased focal length, not to the larger negative size (which actually increases the DoF, not not enough to compensate for the longer lenses).

-- Alan Gibson (Alan.Gibson@technologist.com), December 11, 1999.

Hmmm! What a circle of confusion! I'm suprised this argument hasn't ground to a full stop. (puns entirely intended)

Depth of field is basically governed by the angle subtended from a point on the subject to the diameter of the lens aperture; a smaller angle giving a greater DOF. The smaller the physical size of the aperture (regardless of focal length) the greater the DOF. Yes, format and therefore acceptable "circle of confusion" affect DOF, but in a linear way. Focal length and physical aperture size affect DOF as the INVERSE SQUARE. i.e. For a given f-number (say f/5.6), if you halve the focal length the DOF increases four-fold, but if you halve the format size you only halve the DOF.

As a further aside to all this, because digital cameras can't see anything smaller than one pixel in size, there is a "quantum leap" in focus when the circle of confusion gets smaller than the size of a pixel, so lower resolution cameras effectively have greater DOF.

What this all amounts to in reality, is that 10mm focal length lenses give you next to no control over depth of field. You get everything in focus, whether you want it or not.

-- Pete Andrews (p.l.andrews@bham.ac.uk), December 16, 1999.

Thank you, Pete, for an excellent explanation. I suspect you may still get an argument. I gave up.

-- Steve (milwaukeechrome@aol.com), December 16, 1999.

Sure, Pete is correct. If you halve the format size, the DoF will also halve (because the acceptable diameter of the circle of confusion also halves). If you simultaneously halve the focal length (to maintain a constant angle of view), this will multiply the DoF by four, which more than compensates for the format size halving the DoF.

-- Alan Gibson (Alan.Gibson@technologist.com), December 17, 1999.