The most common problem - DoF

[cgipson1]

Hey - need some clarification...

so I'm using this online DOF calculator...

Online Depth of Field Calculator

it mentions "Use the actual focal length of the lens for depth of field calculations. The calculator will automatically adjust for any "focal length multiplier" or "field of view crop" for the selected camera."

What does it mean by actual focal length? In the picture next to it on an 18-55 it has 55 circled. Does that mean I will use the 55 no matter what focal length i'm using?

It means if you are using a 50mm... then user 50mm. If you are using crop body, you might be tempted to put in a 75mm... but don't!
Use the ACTUAL FL.... not the FOV (crop sensor) FL!

Does that help?

 

[ceejtank]

Hey - need some clarification...

so I'm using this online DOF calculator...

Online Depth of Field Calculator

it mentions "Use the actual focal length of the lens for depth of field calculations. The calculator will automatically adjust for any "focal length multiplier" or "field of view crop" for the selected camera."

What does it mean by actual focal length? In the picture next to it on an 18-55 it has 55 circled. Does that mean I will use the 55 no matter what focal length i'm using?

It means if you are using a 50mm... then user 50mm. If you are using crop body, you might be tempted to put in a 75mm... but don't!
Use the ACTUAL FL.... not the FOV (crop sensor) FL!

Does that help?

It does. However my last question on it is. so If I'm using my 70-300, and I have it extended to 200mm on the lens, I should enter 200MM in it? Or would I put 300 in, like they circled in that picture.. I know its a really stupid question, but for some reason my brain just isn't grasping it and i need it spelled out like a 5 year old..

 

[TCampbell]

The depth of field calculator doesn't care that you have a zoom lens capable of a whole range of focal lengths. They want to know what focal length you are using for the shot in question (the shot for which you plan to calculate the depth of field.)

So if you' are using your 70-300mm and you've got it zoomed to the 200mm length then you'd enter 200mm as the focal length (ignore the rest of the range.) ALSO... don't confuse the focal LENGTH with the focused DISTANCE (two totally different things.) E.g. you can use your 200mm lens and focus on a subject 50' away. There should be a different box to enter your subject distance in the calculator.

 

[TCampbell]

And just to provide an example which should visually emphasize, these are two examples of Depth of Field using a macro lens.

The nearer you are to your subject, the narrower the DoF will naturally become (even without changing the focal length or f-stop). When you get to the macro range, the DoF can become paper-thin.

To illustrate, consider these two images. I used a surface that makes it easy to see the DoF.

Both of these were taken using a 100mm macro lens. The image on the left was shot at f/2.8. The image on the right was shot at f/32. Notice the f/2.8 shot is offering less than 1 mm closer to the focused point and about 1-1/2 mm behind (so the total DoF is approximately 2mm deep in this shot). The f/32 shot has most of the distance in-focus but if you look at the extreme top and bottom of the photo you'll see that even at f/32 the focus is started to degrade (so the total DoF is just over 2cm deep at f/32 using this focal length and distance.)



Just in case you're wondering... these DoF and blur on these images is natural. I realize it does look a bit fake on the left, but that's an untouched image (other than the fact that I just noticed Aperture added my watermark (whoops... I didn't mean to do that.))

 

[ceejtank]

The depth of field calculator doesn't care that you have a zoom lens capable of a whole range of focal lengths. They want to know what focal length you are using for the shot in question (the shot for which you plan to calculate the depth of field.)

So if you' are using your 70-300mm and you've got it zoomed to the 200mm length then you'd enter 200mm as the focal length (ignore the rest of the range.) ALSO... don't confuse the focal LENGTH with the focused DISTANCE (two totally different things.) E.g. you can use your 200mm lens and focus on a subject 50' away. There should be a different box to enter your subject distance in the calculator.

Thats what I thought - but the way they phrased it / the number they circled for it confused me. Thats what I couldnt wrap my head around. Thanks for the clarification sir.

 

[KmH]

Online DoF calculators worth using have you specify the camera model, or a crop factor modifier, that makes the necessary adjustments to the calculations.

 

[spark123]

Thank you, have got that in mind.

 

[Patrice]

These DOF calculators are pretty handy as learning tools, however I fail to see the everyday in the field utility. Most modern DLSR's have a DOF preview function and nearly all digital cameras now have a handy little screen on the back to review your image and ensure the settings were adequate for the image in mind. All my older lenses, even an older push/pull zoom, have DOF scales on them. These were very handy when it took a fair while before you could review your images.

 

[timor]

Depth of field, or DoF, determines a range of distances from your camera that will be have acceptably sharp focus.

Any optical system can focus one and one distance only. DoF is about acceptable sharpness and not about "having everything in focus". There is one more aspect of DoF not mention in this quite nice online manual: the size of intended print which means that one can not magnify the picture indefinitely and still hope that it will be sharp where intended (apart from the lens and material resolution and aberrations).Longer viewing distance with bigger print may "restore" perceived sharpness to some point, it will not help with aberrations. Of course if one has intention to print small (like 5x7) or display the pictures on the computer screen only, there is no point to worry.