Quick question on DOF calculator

[zulu42]

When I measure camera to subject distance, is the measurement taken from the subject to the film/sensor plane? If so, how does one measure? I've been using a guess of about on half inch forward of the rear LCD. Close enough?

 

[dxqcanada]

Flange focal distance - Wikipedia

most cameras should have a mark on the top to mark the sensor/film plane

circle with a line through it

 

[zulu42]

Thank you

 

[Derrel]

Yup, dxcanada has got it right, the focal plane indicator has long been a small circle with a line through it. Usually found immediately to the left or right of the pentaprism housing, kind of toward the back of the camera, rear edge.

 

[Derrel]

At longer distances, such as beyond 5 feet, an approximation of 1/2 in forward of the LCD is probably accurate enough.

For close range and pseudo-macro type scenarios, you really ought to be measuring from exactly the focal plane, since depth of field is often measured in 1 or 2 cm units.

 

[zulu42]

Found it. Thanks guys. I can only wonder how many times I looked at that little circle...

There have only been a few times where I've wanted to be accurate enough to actually measure. I think I remember reading that some of the mirrorless/EVF bodies will display the DOF calculations. That's a neat perk. I don't see any reason a DSLR couldn't have that feature.

 

[JBPhotog]

DoF is also based on the CoC for that sensor. There is a simple app DoF calculator called Field Tools that lets you input FX and Dx bodies with various lenses. Handy for in the field on your iDevices.

 

[WayneF]

"Focus distance" (like say 20 feet) is measured to the sensor plane. And for example, when the lens specification mentions "closest focus" distance, it is to the sensor plane. Even for a macro lens, when we are more interested in the "working distance" in front of the lens, but the closest focus spec measures to the sensor plane.

However, Depth of Field and Field of View calculations use the Thin Lens model, which computes to the focal node, probably inside the lens somewhere (but just in front of lens for a telephoto lens, and behind the lens for a short wide angle lens). This is likely relatively not all that many mm from the sensor plane, which really won't be important at all at 10 or 20 feet to the subject. But it definitely matters up close.

 

[fmw]

I think the best DOF calculator is the monitor screen on the back of your camera.

 

[Derrel]

I think the best DOF calculator is the monitor screen on the back of your camera.

Yup. Shoot. Review. Adjust as needed.

 

[WayneF]

Ooooh. With regard to judging DOF, there is a huge problem with that logic. It violates the basics. Viewing size really matters greatly for DOF.

The CoC limit for DOF is judged for viewing a standard size comparison print. That standard print size for DOF considerations has always been an 8x10 inch print viewed at 10 inches (25 cm). That may not be everyone's standard viewing size, but it is the standard that DOF calculation is designed for, and what DOF calculations report.

Because, when we enlarge the viewed print, we can of course see small detail better and can detect more blur in it. The more we enlarge it, the blurrier it gets, but viewing it tiny will look pretty good. This is the reason why small sensors specify a smaller CoC than a larger sensor, simply because they must be enlarged more to be the same viewing size (of the standard 8x10 inch print). For example, 35 mm film (crop factor 1x) normally has an 0.03 mm CoC, and a 1.5x crop factor camera will have CoC of 0.03/1.5 = 0.02 mm (because it must be enlarged 1.5x more).

Novices rarely know about this standard viewing size aspect of DOF, but here are a few links that mention the 8x10 comparison size for DOF judgment.
depth of field standard 8x10 size - Google Search

The camera rear LCD is certainly extremely useful for judging our photo result. But unless you are using an 8x10 inch view camera, it seems about useless for matching the standard 8x10 inches that DOF calculations are designed for. It is true though, if you detect DOF blur on the small rear LCD, you are way past the limits for a larger print (unless of course you only intend to ever view it at LCD size).

 

[JBPhotog]

Additionally to what @WayneF summed up nicely. If one is seeking the actual DoF then using a calculator that specifically identifies the sensor size, CoC, focal length and f-stop is the only accurate method. My experience shooting product over the past 40+ years in all formats tells me that knowing the physical width in inches or millimetres of the DoF is an asset.

Back when cameras actually had DoF preview buttons that functioned well with the focussing screen, a visual was possible but for critical work measurements are required. This is supported by the ever present whinging from many people of the lack of DoF indicators on modern lenses.

 

[Derrel]

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• Use the magnify feature dude. Get up to speed on modern working methods. Using the magnifying feature has been standard operating procedure for about 10 years now. With today's high-quality LCD rear screens you can view the equivalent of a 40 by 60 inch or larger final image quite simply by pressing the magnify button several times. You can scroll around the image and make what are easy decisions. Your logic sounds good (on paper , or to the unititiated) but it is way out of touch with reality. There is no as you say huge problem with this method.

 

[JBPhotog]

Magnifying an image on the LCD has lots of caveats when attempting to determine accurate DoF. Anything over 100% will introduce pixelation compounded by the fact the LCD already exhibits RGB pixel issues. My 5k monitor easily out resolves the LCD on the back of my cameras.

You can determine the plane of focus on the LCD but knowing exactly where the boundaries of focus are very difficult, a ruler or measuring tape is way more accurate.

 

[Derrel]

If you have an old crap LCD screen you cannot evaluate your images well enough. If however you have a new camera with a 3 inch or larger LCD screen with more than 1 million pixels 3 to 5 presses of the magnifier button will 1 give you an image that is clearer than Wayne's theoretical 8 by 10 in print View at 10 in. A

In many cases,we can easily view on-screen images larger than Wayne's tiny 8 by 10 inch print standard . In 2007 I bought an Apple 30 inch Cinema display.... the images viewed on this screen are larger by quite a bit than an 8 by 10 inch print. In Wayne's example ,apparently to actually evaluate depth of field you must print out an image and view the print at 10 inches to make your depth of field judgement. That sounds like a great working method for 1940, but it is now 2020, and my Nikons shoot images straight out of camera that are about 68 in tall at full.size. Using early Early-era c digital single-lens reflex cameras it was not possible to magnify the image very much, but if you shoot raw plus JPEG and have the sharpening turned up to medium or higher, using the LCD screen on the back is quite sufficient to determine depth of field at the time of actual shooting. If you use a new modern Sony A7 with a 47 megapixel sensor and a big beautiful screen it is Child's Play to make actual depth of field decisions. Like I said it is no longer 1940 and the ages-old standard of depth of field being judged by a printed 8 by 10 viewed at 10in is kind of like riding in the rumble seat of a Model A Ford....today many (most) images are not printed but are viewed on screen, either quite small, or fairly large, or even as large as a 70 inch wide TV.

If the depth of field standard is the 1940 era 8 x 10 in printed image viewed at 10 in, then how the heck are we supposed to determine if we have adequate depth of field for an image which will most likely be seen no larger than 4 inches on the diagonal on a small cell phone? And what are we supposed to do when we are making images that will be seen on a very large high resolution 5K Apple Retina display? The idea that one cannot determine the depth of field by using the magnifying feature on a modern high resolution LCD rear screen is absurd. 20 years ago, no we could not do it because screens had around 300,000 pixels in total. Today even an average LCD screen has 921000 pixels