FLYMPH FORUM

Depth of Field, Aperture and Sensor Size: DSLR and Compact Digital Cameras

NOTE: This post is followed by several more that offer more accurate information - READ BELOW

Okay, many of us use a compact digital camera and have some questions about how the aperture settings or F/stops compare to the DSLR cameras. I was under the assumption that we were using terms that translated directly, but there are some differences that lead to some confusion. Unless you read your manual... you may not have realized these differences.

Ray was kind enough to send me a copy of Bryan Peterson's Understanding Exposure. He does a nice job describing how these things work in a way that I can understand. He describes how the compact cameras lenses rate their zoom lens' focal length as 7-21mm, 9-72mm or 9.7-48mm. The SLR lens has the focal length written on each interchangeable lens. He says "...the 7-21mm fixed lens is equivalent to a 38-155mm zoom lens on a DSLR. The 9-72mm is equivalent to 35-280mm and the 9.7-48.4mm is equivalent to a 38-190mm zoom lens. Setting the depth of field using the aperture setting depends on the lens' focal length." The numbers above explain a lot about how the compact cameras get such fantastic depth of field range, but have a very tough time getting a smaller depth of field to help blur the background in portraits or shooting macro shots where you want the background to fade away. He says "...when you set your lens to f/2.8 - an aperture of f/2.8 is equivalent to an aperture opening of f/11 on an SLR. ...when you're at f/4, you're able to record a depth of field equivalent to f/16. At f/5.6, you're equivalent to f/22. At f/8 you're equivalent to f/32..." and so on. So when I describe using an aperture of f/12 for a particular shot, if you were under the impression, as I was, that this translates to the compact camera's aperture setting, it's quite different. My Canon compact camera could get an f/stop of f/8 created a deep focal range. He doesn't describe what happens when you set your camera to it's "macro" setting, but the effects might be different. There is a learning curve in shooting with the DSLR as the compact lens' actual aperture is greater (higher number) than its description and the shutter speed is faster as a result. I've always used a tripod for shooting macro fly pics, but the shutter speed on the DSLR is much slower when trying to gain more depth of field the tripod is critical. A remote trigger is necessary or using the delay release as I have done on with compact camera. This is all very technical, but for some of the earlier information posted here to be useful, it's something to keep in mind.

If anyone, most of you, have a better understanding of how this works, and how it might help us, please add or correct whatever you think is necessary.

w

William,

The above explanation does not work for me. I understand focal lengths (which relates to lens optics), and I understand sensor sizes (they have a width, a height, and as a result of these two they have a diagonal). I am not sure where fixed lens vs. interchangable lens influences the focal length.

You may find this url of use. If nothing else, it has a nifty conversion function which allows you to enter a focal length and a sensor size, and spits out the equivalent focal length in 35mm film terms.

http://www.cambridgeincolour.com/tutori ... r-size.htm

For example, entering a focal length of 10mm in a 1/2.3" sensor camera results in a 35mm film equivalent, i.e. a full frame DSLR sensor, focal length of 56mm.

Cheers,
Hans W

The above site also provides an online DoF calculator you may find informative:

http://www.cambridgeincolour.com/tutori ... ulator.htm

Cheers,
Hans W

This is all really interesting stuff.
I own what is called a "cropped-sensor" DSLR camera. It's a Canon t2i (or 550D). Most entry level DSLR's are cropped-sensor. In this case, if you are using a 50mm lens, for example, you multiply that by 1.6 to get the 35mm camera equivalent. So, for a 50mm lens, the actual focal length would be 50 X 1.6, or 80mm. With a "full-sensor" camera (usually much more expensive), the multiplier would be 1.
Lots of this stuff is still floating above my head somewhere, but the learning process is a fun journey.

Not high tech, but effective: When Don Bastian is tying a fly specifically for a photograph, such as for an article or book, he bends a length of fine wire at 90 degrees and then ties one leg to the backside of the hook shank while he's tying the fly. When he's finished, he has a fine wire post protruding from the fly that can be stuck into a background surface to "suspend" the fly in what appears to be mid-air.

Hans, I'll revise this when I have time to read the links you've provided and have some working knowledge of the topic. There was some confusion earlier and I thought quoting a professional on the differences between compact camera lenses and those on a dslr would help, but apparently not. This is partly why I prefer a dialogue format, as in the forum, to a publication of my knowledge or presenting a final product. My knowledge would easily fit on a handy note card...but would not be of much use.

Rodger, sensor sizes on our individual dslr's is another matter, up to each of us to understand our own conversions. That's a simple formula that I do understand but it's not a general topic that applies, except where we are discussing lenses and their focal length. My attempt at clarification was regarding the aperture size indication and the differences between compact cameras and dslr's. Getting to know my own camera's issues is my burden to sort out for myself. Thanks for your help.

Bill, I have never heard of building in part is the mounting into the fly construction which is brilliant when flies are tired for shadow boxes or just for presentation. I dint know if Don thought of it, but kudos to him for an innovative solution. I only tie fishing flies but I can see where this technique might have an application sometime. Wheels are turning.

William Anderson wrote:Rodger, sensor sizes on our individual dslr's is another matter, up to each of us to understand our own conversions. That's a simple formula that I do understand but it's not a general topic that applies, except where we are discussing lenses and their focal length. My attempt at clarification was regarding the aperture size indication and the differences between compact cameras and dslr's. Getting to know my own camera's issues is my burden to sort out for myself.

William,

The DoF calculator, second url, gives you precisely that understanding. It corrolates what you get for a given aperture, using the focal length and camera sensor size you are using. What it also provides is handle on why it requires a much higher f-value to get the same DoF on a larger sensor/longer focal length compared to a smaller sensor/shorter focal length combo.

Note - whether it is a fixed or interchangable lens does not enter the scenario - other than as a given that most point and shoot 'fixed' lens cameras tend to have smaller sensors (1/2.3", 1/1.7", 1") - as you have discovered, they also top out at f8 for most models. The camera I use I can tweak to f10.2, which is one of the reasons I like it - for the DoF gained by that extra 3/4 of a stop.

Larger sensor cameras tend to be found in interchangable lens systems (1", m43, aps-c/DX, and the 35mm equivalent FF), but also for those sensors there are cameras with a 'fixed' lens for all but the FF size. The interchangable lenses tend to have much higher max f-value settings on board, typically f22 or f32.

For any given sensor/focal length combination - the higher the f-value set, the more DoF will be had. Sounds good, sure, but things are not quite that simple. Enter 'lens diffraction' - with as a result a softening of the image, becoming less sharp.

Third url for you:

http://www.cambridgeincolour.com/tutori ... graphy.htm

Bottom line - find the aperture sweet spot for your sensor/focal length combination, with the optimal balance is struck between a sharp undistorted image and DoF.

Cheers,
Hans W

Hans, I sincerely want this entire thread to be as valuable as possible to all our forum members, and I'm doing my best to create a balance between general topics like lighting, composition, backgrounds, set-up, etc. How we use and what we understand about how our cameras work is essential. I'm not the least bit bothered by being corrected, frequently, and I can't tell you how much I appreciate your input and steering these discussions toward the best information available and helping me understand all the aspects involved. I've avoided some of the issues that get to technical because I don't have the range of knowledge yet to explain them and because they may not interest many readers. However, your explaination and the links above are awesome. They are a really great resource. I fooled with the conversion link and read through the others. For anyone interested in taking their photography to a higher level, this is very helpful. Thank you.

Hans, a quick question. You stated above "I am not sure where fixed lens vs. interchangable lens influences the focal length." I shoot, both on my compact camera and my DSLR in aperture priority mode. If the equations that determine the depth of field for both types of cameras result to little difference, this is a non issue, but, my purpose for raising the topic to begin with was that in many of the early photographs I discussed the f/stop used resulting in various depths of field. I wanted to clarify any confusion if others were looking at those numbers and expecting similar results. I didn't want to be posting misleading information. When I post comparative images with stated f/stops, I thought that information would be useful, but if conversions are necessary for each individual camera, those stated f/stops are of little value to the reader, other than to describe results with higher or lower f/stops or aperture settings. Is that clear as a Spring spate? It might appear the lunatics are running the asylum. Thanks again.

I've updated the LINKS post, page 1, to include these more technical aspects. I'll continue to make edits, adding headers throughout to help create a more cohesive and useful thread.

William,

The variables are sensor size, focal length and aperture used, distance to object - these four determine the DoF achieved. Whether it is a camera with a non-interchangable lens or a body with an interchangable lens - the DoF knows nothing of the camera. All I tried to do with my observation is to eliminate the suggestion (as I read it) that somehow this (fixed/interchangable) impacts the outcome - it does not. Only the four variables listed do.

For an image where you like to provide technical info on the shot, the relevant information I would list is camera make/model (which gives indirect info on sensor size, and also gives info on the sensor rez), focal length and aperture. The distance to the fly is much dependent on its size, but also whether you have cropped the image and if so how much.

Does that make more sense?

Cheers,
Hans W

It does. I believe this level of detail is beyond the scope of this thread, although some may want to investigate further and will find the links provided very useful. Is the initial post at the top of page 9 nonsense? Should I remove it to avoid any further confusion? As to what to include in the information provided with a given image, I believe I have tried to do that, listing the lens (the camera is listed in the beginning of the thread) and the aperture, noting that shutter speeds and ISO numbers are invalid due to post-processing.