Bmac Loc: Long Island, NY

I was wondering what the correct focal length to post for photos taken. In other words, should I state the focal length with the camera's sensor taken into consideration? Thanks

MT Shooter Loc: Montana

Your choice. Your EXIF data will state the actual focal length as well as the 35mm equivalent focal length so that info is always available to you even in mid-zoom, easy to note both if you want to.

Bmac Loc: Long Island, NY

Thanks MT, I just didn't know what, if any, the protocol was.

snowbear

I post the focal length of the lens. I feel there is less confusion that way, especially since I also shoot 35mm film.

Bmac Loc: Long Island, NY

Thanks Snowbear, makes sense, guess I will continue posting that way. I'm assuming most people do.

Ragarm

The answer depends, as always, on what you want to communicate. Ask yourself, why would I post any reference to the lens? The only logical reason would be to provide information about the shot.

What kind of information? Something that indicates exposure and layout. What else is important?

OK, let's take layout. If you carefully arranged artificial lighting devices in order to achieve an effect, you'd want to indicate that. If you took advantage of natural light sources in a unique or non-obvious way, you'd want to indicate that. What's special about the layout that enabled you to get that shot? That's what you want to indicate.

Now, exposure. Post the three corners of the exposure triangle, because that is what dominates exposure (ISO, aperture, shutter speed). If you used anything special to achieve the effect, such as filter, camera, sensor, processing, film, etc., then that would be pertinent.

Now, to the crux of your question. You'll notice that focal length underpins both layout and exposure. Because cropping has always been so commonly employed, and because it alters the apparent focal length, it has been common to specify it along with the three corners of the exposure triangle.

But which focal length? Would you specify an ISO different from what characterized the sensor? Would you specify an aperture or shutter speed different from what affected the amount of light passed through the lens to the sensor? No, of course not. So, why would you specify a focal length different from what affected the light density on the sensor?

What is the focal length that affects the light density on the sensor? It is either the FX (marked on most lenses), or the DX (sometimes marked on lenses made for specific DX sensor format cameras). It is important to understand that the conversion factor between FX and DX isn't standard, but depends on the camera manufacturer.

So, the only way to be unambiguous about exposure is to specify the focal length for your lens/camera combination, this along with the three corners of the exposure triangle, and to augment that with pertinent non-standard or uncommon devices used. Otherwise, why bother at all?

Of what use is posting the focal length printed on your lens, without telling the rest of the story? It seems to me that we should append our focal length specification with eq., to indicate the equivalent focal length of our lens/camera combo.

Bmac Loc: Long Island, NY

Hmmm, okay so you feel the bottom line is as follows. I use a Nikon D80 so when I take a picture with a 300mm lens at full focal length it's actually 450mm after the sensor crop. So I would post that the photo was taken at 450 rather than 300? Is that correct? I was wondering how most people post the focal length. It seems that they don't convert it to their particular sensor. I guess the reason for my question was if everyone was doing it a certain way that's the way I would do it. I'm beginning to confuse myself. :) :)

patrick28 Loc: Port Jeervis, NY

If the post concerns the photographic process, technique or equipment -- as opposed to the art, emotion or content -- I would like to see the technical data we're talking about. At a minimum, camera, lens, S, A, ISO and focal length.

Yes, I know it's in the EXIF but many of our members are not familiar with that information or how to access it.

I get exasperated at posts such as, "I came across this phenomenal feature while out walking this morning." + photo.

"I can't believe how effective this technique (feature, edit, sw, trick,) is."

OK, we agree. Now SHARE IT!
That's what this forum is all about.
Enough with the strokes.
"Just the facts, Ma'm!"

Ragarm

I post the equivalent focal length. The confusion you voice is the very reason why everyone should.

Take my D7000 for example. It's kit lens is a DX lens, so I'll post the focal length I read on it, the same it reports to the camera. However, I use two other lenses that carry FX calibrated numbers and report these to the camera. Of what use is it to you to know that I shot at 100 mm without also telling you which type of lens I used?

There should be no confusion. There is only uncertainty about what people report. There are numbers that mean something and there are numbers that don't. Join the ranks of us who use numbers that mean something.

In any conversation, don't you ask for clarification when someone tells you something ambiguous? Don't you try to be clear when you tell people things? This is not different. Let's be clear about what we report, and let's ask for clarification when confronted with ambiguous reports from others. We'll all be better off.

Ragarm

Yes, my feeling exactly!

You can post to show off while selfishly hiding your technique, or

You can post to show off while generously revealing your technique to others for everyone's benefit.

I prefer the latter. The former is certainly easy enough to recognize.

Adirondack Hiker Loc: Southern Adirondacks

Here is the rest of the story: 15 mm is 15 mm whether it is FX or DX. This focal length, as well as the f/stop define the depth of field, that is why the info is critical. For me, f/13 and 12 mm says everything from 18 inches to infinity will be sharp, if I focus at 3 feet. Cropping does not change it one bit.

Ragarm

We don't disagree about what is important to you on your camera. However, if I want to understand how YOU did that nice shot, I'd want to know if your 15mm focal length was effectively 15 mm in an FX system or 22.5 mm in a DX system such as mine.

That might not make a significant difference with regard to depth of view, although it does make some, but it does affect exposure. My lens will gather significantly less light at f/13 and fl=22.5mm (effective) than yours did at f/13 and fl=15mm (effective).

Ragarm

Your statement appears to conflict with this, from http://www.cs.duke.edu/~parr/photography/faq.html#35mmequiv

"Every lens has a focal length which is a physical property of the lens. Once the lens is made, this cannot be changed. The field of view associated with a lens will be a function of the area projected by the lens that is captured by the camera. For 35mm film photography, this area is 36mm by 24mm. Note that if you change the area captured, the field of view also changes. For example, using a 50mm lens on an APS format camera yields a very different field of view than using 50mm lens on a medium format camera.

For many years, amateur photographers who used 35mm film and no other systems became accustomed to associating particular focal lengths with particular fields of view. When these photographers moved to other systems, such as medium format, or digital, it was sometimes convenient to think about lens focal lengths in terms of the equivalent field of view they offered in the more familiar 35mm film world. "

Also, I refer you to http://en.wikipedia.org/wiki/Aperture
where it is explained that the light impinging upon the sensor is proportional to focal length. Clearly, the relationship between lens focal length and sensor characteristics plays a vital role in exposure.

See also http://forums.dpreview.com/forums/read.asp?forum=1030&message=31367179
where the conversion between DX and FX focal lengths is explained.

Ragarm

You might also be interested in this from Nikon:
http://support.nikonusa.com/app/answers/detail/a_id/288/kw/dx%20focal%20length/related/1
The D700 has the FX and DX option. All other Nikon DSLR cameras utilize the DX sensor.

FX- uses the entire 23.9 x 36mm sensor to utilize almost all Nikkor lenses for image capture speeds of up to 9 frames per second.
DX- uses a 15.9 x 23.7mm portion of the sensor to allow the use of Nikon DX lenses, and achieve maximum capture speeds of up to 11 frames per second.

5:4- uses a 30 x 24mm portion of the sensor, and is the choice for easy output print sizes of 4"x5" and 8"x10".

* If we multiply the 35mm focal length by 1.5 we can get an approximation of the "digital" angle of view for the DX format.

Adirondack Hiker Loc: Southern Adirondacks

I do not see any conflict at all, f/stop and focal length are the key factors. Sensor size plays a role, but it actually works against you in landscape photography. That is why my instructor, a well published photographer, uses only DX bodies. It boils down to field of view. Let's compare my D5000 (DX) to a D700 (FX). At 10 mm I have the same field of view as a 15 mm lens on an FX format. BUT, because I am using the shorter focal length lens, my hyperfocal distance at f/11 (my common f/stop) is 1.5 feet compared to just over 2 feet for the same field of view as the D700. Not much of a difference, but still and improvement. In landscape photography, foreground rules, so you want every inch of improvement possible.