Depth of field

[bmattock]

http://en.wikipedia.org/wiki/Grok

Grok (pronounced grock) is a verb roughly meaning "to understand completely" or more formally "to achieve complete intuitive understanding". It was coined by science fiction writer Robert A. Heinlein in his novel Stranger in a Strange Land, where it is part of the fictional Martian language and introduced to English speakers by a man raised by Martians.

Yes, you have sussed me out. 1960's dreamer, scifi recidivist, and general man-about-town. I am that merry wanderer of the night.

Best Regards,

Bill Mattocks, Cavalier Mal Fet

 

[jaapv]

Fond memories- I'll pull it out and reread it tonight Haven't looked in that one for -twenty??- years. 🙂

 

[dnk512]

Thank you guys! This was the most educational and perfectly behaved argument I have heard in many years. Thank you again!

BTW, for those thinking of getting an SLR (or other ground glass camera) to play around with the focus zone: The quality of the out of focus image (pleasant or distracting) may not be accurately judged by the image on the focusing screen. Only the final image (print/screen) can be used to do so. Knowledge of the out focus properties of a lens (front/back, wide open/stopped down, near/far, high contrast edges, foliage etc) will help one toward mastering that lens.

 

[bmattock]

Thank you guys! This was the most educational and perfectly behaved argument I have heard in many years. Thank you again!

BTW, for those thinking of getting an SLR (or other ground glass camera) to play around with the focus zone: The quality of the out of focus image (pleasant or distracting) may not be accurately judged by the image on the focusing screen. Only the final image (print/screen) can be used to do so. Knowledge of the out focus properties of a lens (front/back, wide open/stopped down, near/far, high contrast edges, foliage etc) will help one toward mastering that lens.

This is exactly what the oft-ignored "Depth of Field Preview" button or function is for - to momentarily stop down to the actual aperture you intend to use so you can see what the DOF will look like.

However, as you say, this is only an approximation - primarily because at the smaller apertures (larger numbers), the screen may well become too dark to view very well.

I agree with you also that the best judge is experience in this case. But the SLR ground-glass (or the 4x5, even upside down and backwards) can be a useful tool for getting past the 'visualization' part.

Best Regards,

Bill Mattocks

 

[JoeFriday]

my thoughts regarding DOF is that it should be used if you're emphasizing a particular person.. especially if s/he is in a group.. I rarely try to employ DOF in street photography.. and avoid it for landscapes.. but it can be very effective for isolating specific outdoor elements.. leaves on a tree.. cattails in a field.. that sort of thing

 

[Theo-Prof]

Just to point out - the larger the format, the shallower the DOF for any given focal length and aperture combination. . . . So a LF, as William points out, will have very little DOF for the same focal length and aperture as a 35mm camera.

Best Regards,

Bill Mattocks

Bill, I hate to enter late in the discussion, but I have to disagree with your explanation of relative DOF between formats. A 90mm lens on a 4x5 camera has the same DOF as a 90mm on a 35mm camera. However, a 90mm on a 4x5 is a wide angle while a 90mm would be a short-tele on a 35mm camera. Their relative fields of view are different but not their DOF.

If you mean that a "wide angle" on a 4x5 has less DOF than a "wide angle" on a 35mm, then I completely agree. However, this is because the larger formats have longer focal lengths to achieve the same angle of view as the smaller formats. So, (subject distance and all else being equal) DOF is relative primarily to focal length, the larger formats have less DOF because they require longer focal lengths than the smaller formats in order to achieve the same field of view. This is also why compact digicams have such deep DOF, since their sensors are so small they require VERY short focal lengths in order to achieve a given angle of view relative to, say, a 35mm camera.

Kevin

 

[bmattock]

Bill, I hate to enter late in the discussion, but I have to disagree with your explanation of relative DOF between formats. A 90mm lens on a 4x5 camera has the same DOF as a 90mm on a 35mm camera. However, a 90mm on a 4x5 is a wide angle while a 90mm would be a short-tele on a 35mm camera. Their relative fields of view are different but not their DOF.

If you mean that a "wide angle" on a 4x5 has less DOF than a "wide angle" on a 35mm, then I completely agree. However, this is because the larger formats have longer focal lengths to achieve the same angle of view as the smaller formats. So, (subject distance and all else being equal) DOF is relative primarily to focal length, the larger formats have less DOF because they require longer focal lengths than the smaller formats in order to achieve the same field of view. This is also why compact digicams have such deep DOF, since their sensors are so small they require VERY short focal lengths in order to achieve a given angle of view relative to, say, a 35mm camera.

Kevin

Kevin,

You're absolutely correct, and not the first person to point that out in this thread.

And I mean no disrespect...being correct is important...

The one word I *should* have inserted to alleviate confusion is 'equivalent'. The focal length *equivalent* to a given focal length in 35mm will have the same angle of view (that's why it is equivalent), but it will have a different DOF, because DOF is a function of the *actual* focal length, not the *equivalent* focal length.

Yes, 90mm is 90mm is 90mm and will have the exact same DOF characteristics no matter what size media the image it passes is being recorded upon - I agree. But as you point out, 90mm is a mild telephoto on a 35mm format - a wide-angle on a 4x5 LF camera, and an extreme telephoto on a digicam with a tiny digital sensor.

A 90mm *equivalent* on each format will possess the same angle of view but different DOF characteristics - and this is what people notice, especially new photographers. I think it is more important to explain why in a way that makes sense and can be demonstrated than to be technically correct and introduce concepts that go way beyond the simple question.

Again - I think this is where the techies turn off the newbies. We techies grab our beanies, give the propellers a spin, and launch off into discussions about circles of confusion, point-sources, and other esoterica - which matters to us and is far more 'correct' than simple explanations that shade facts. But it comes off as utter rubbish and confusing nonsense to the newbie who just wants to answer a simple question - what's the deal with DOF?

My goal is not to turn off the newbies, but to answer their question in terms that are simple, make sense, and if they are not technically correct - at least they present concepts that can be tested and do work. The technical correctness police can stun-gun me later.

Best Regards,

Bill Mattocks

 

[pvdhaar]

To put things in perspective..

To put things in perspective..

To put things in perspective...

my first twenty years of photography were spent with the shutter of the Zenit set to a solid 1/30. The aperture followed from what the light meter read, or from the distance table on the back of the flash. DOF was merely a byproduct that I didn't have a clue about, it just happened..

Can you imagine that? Twenty years.. and I took tons of nice pictures..

 

[jaapv]

Bill, I hate to enter late in the discussion, but I have to disagree with your explanation of relative DOF between formats. A 90mm lens on a 4x5 camera has the same DOF as a 90mm on a 35mm camera. However, a 90mm on a 4x5 is a wide angle while a 90mm would be a short-tele on a 35mm camera. Their relative fields of view are different but not their DOF.

If you mean that a "wide angle" on a 4x5 has less DOF than a "wide angle" on a 35mm, then I completely agree. However, this is because the larger formats have longer focal lengths to achieve the same angle of view as the smaller formats. So, (subject distance and all else being equal) DOF is relative primarily to focal length, the larger formats have less DOF because they require longer focal lengths than the smaller formats in order to achieve the same field of view. This is also why compact digicams have such deep DOF, since their sensors are so small they require VERY short focal lengths in order to achieve a given angle of view relative to, say, a 35mm camera.

Kevin

Guys, let's get our facts straight before we start saying the same thing and calling it disagreeing. To stop the beanies turning I'll call Cicles of Confusion "picture dots"OK?. Yes, the DOF characteristics of any given lens are unchangeable by the format if we use the same enlargement. This is incontestable.However, as different formats require different enlargement, the requirement for the size of the picture dots, that create the DOF is different. The picture dots being circular, the change is related to the diagonal of the format. So if one puts a 90 mm lens on a RD1, with a crop factor of 1.5, it will turn into a 135 mm equivalent, but the apparent DOF, assuming a similar, thus more enlarged, print will be the root of the crop factor, turning the DOF into a 122 mm lens, about.

To be difficult, a film will show up differently from a sensor on such small things as picture dots. Film will exhibit diffusion and diffraction in the emulsion layer, a sensor less so. The definition will be dependent on the size and quality of the sensor photosites, but the DOF on a digital photo will be more defined. So our example 90 mm lens will behave like a 115 mm lens on the RD1, DOF-wise and like a 135, field of view wise.