# How to simulate different focal length and sensor size on an image?

I like to simulate different focal length and sensor size based on an image i have taken. I have a full frame body so I like to know how the picture would look like if I have taken with a cropped frame body. Also a different lens. For example, if I have a picture taken with a 200mm lens on a full frame body, I like to know how it would look like using a 400mm lens etc. Is there any software that I can use to simulate that effect? Possibly Photoshop and so how can I do that in Photoshop?

I know Nikon has a Lenses Simulator but I like to see with the image I have taken. The picture is just example and it isn't correctly cropped.

Thanks.

-
Check out the exercise in my answer to What is Angle of View? This covers focal length and sensor size, and once you understand that it's pretty easy to see why the relationship between focal length and crop is linear. – mattdm May 12 '13 at 17:58
If you have a recent Nikon, put it into DX mode to simulate. Doing it after the fact doesn't really show it because the DOF for the same angle of view will be different between the sensor sizes. – John Cavan May 12 '13 at 19:47
No it won't. If you are comparing different sensor sizes using the same focal length, the angle of view changes and DoF stays the same, not the reverse. DoF only changes when you change position to frame the subject the same with a different sized sensor, assuming the same number of ppi are used when printing or viewing on a monitor. – Michael Clark May 12 '13 at 20:09
@mattdm .. very nice answer explaining the Angle of View .. I will take a closer look later .. – lawphotog May 12 '13 at 21:18

If you want to compare what your picture would look like taken from the same position with the same lens on a 1.5X crop body, divide the dimensions of your image by 1.5. Your 2473 X 1649 pixel image would crop to a 1649 X 1099 pixels (2473/1.5=1649, 1649/1.5=1099). To center the crop on the middle of the image, offset the starting point by 1/2 the difference. That figures to 412 to the right and 275 down from the top left corner (2473-1649=824, 824/2=412. 1649-1099=550, 550/2=275).

You would do the same thing to compare two focal lengths. If your picture was taken with a 200mm lens, to see what the same shot with a 400mm lens would look like divide the old lens by the new one (200/400=0.5). Multiply that result by your original dimensions (2473x0.5=1237, 1649x0.5=825). Crop your 2473 X 1649 image to 1237 X 825, using the method outlined above to center the crop, then enlarge it until it is the same size as your original image.

-
Thanks Michael ... that's what I wanted to know .. A good dummy guide HAHA :P – lawphotog May 12 '13 at 21:16

To simply simulate the crop then you can just set the canvas size (Alt-Ctrl-C in Windows) to be the same width & height in pixels as a cropped image.

If you want something more like your example image then then record an action of you drawing a rectangle shape by selecting the shape tool, choosing rectangle and then from the little arrow next to the custom shape you can set a specific size in pixels then place your box on top.

-
Thanks for your answer. But it's more like you are teaching me how to use Photoshop. I know how to use Photoshop alright. What I don't know is how much do I crop to simulate that effect. For example, how much do I crop on a picture taken with 200mm focal length to become a picture taken 400mm focal length? – lawphotog May 12 '13 at 16:20
@LaurenceNyein it's a simple linear relationship between focal length and field of view, to simulate a 400mm focal length with a 200mm image, just half the width and height. – Matt Grum May 12 '13 at 16:26
@LaurenceNyein - The question I answered was the one you posted, specifically " I have a full frame body so I like to know how the picture would look like if I have taken with a cropped frame body." I gave you two ways to directly enter the values in Photoshop as you mentioned using that (one of them not an obvious option for most users) and other existing questions such as the info posted by mattdm and matt grum outline how to work out how big the crop/box dimensions should be. If that wasn't what you wanted to know then feel free to clarify your question or post a new clearer one... – James Snell May 12 '13 at 19:15
Thanks .. I got what I wanted to know .. – lawphotog May 12 '13 at 21:23