frame size question

[Ian Gray]

its probably obvious but I could do with some help with this one. with my minolta 3x-1x lens (similar to the canon mpe-65) attached to my sony a700 with 1.5x field of view I have fixed focal distance of 40mm at 1x1. with the same lens attached to a full frame camera would I have an image half as big again at 40mm, or would I have a fixed focal distance of 27mm. I'm probably being stupid but I don't want to make an expensive mistake.

thanks for any help

Ian.

[matt_xyz]

Hi Ian,

I'm not sure I completely understand what you're asking, but I assume you're wondering how an image taken at lifesize on a full frame sensor will compare to one taken at lifesize on a 1.5 crop sensor. A 1.5 crop sensor is approx 24mm wide, so if you photograph a 20mm wide insect at 1x magnification it will take up 20/24 of the width of the sensor i.e. 83% of the width of the frame.

If you photograph the same 20mm wide insect on a full frame sensor (35mm wide) it will take up 20/35 of the width of the sensor i.e. 57% of the width of the frame.

To put it another way, an image taken at 1x on the full frame sensor will take up the same proportion of the frame as one taken at 0.66x on a cropped sensor. Alternatively, an image taken at 1x on a cropped sensor will take up the same proportion of the frame as one taken at 1.5x on a full frame.

I'm not sure that helps or whether I've told you what you already know! I've used my MP-E 65 with both full frame and cropped sensor cameras and the full frame is certainly useful for fitting larger insects into frame. On a cropped sensor body I will often want to go below 1x but it's not possible with my lens (or yours, I assume?). The downside is obviously that for tiny insects the apparent magnification is greater on the cropped sensor (i.e. a tiny insect will take up a larger proportion of the frame).

Matt

[operanut1972]

Maybe I'm over simplifying this but surely on a full frame sensor it'll be as per the lens? 24x35? On a cropped sensor it's roughly 85% smaller? I'm not sure exactly what your getting at. So ignore me if I'm not making sense.

All the best!

[roseway]

I'm not clear what the question here is either, but the basic point is that the sensor size doesn't affect the focal length or the focusing characteristics of the lens at all. All it affects is the field of view. So, compared to a full frame sensor, a crop sensor will be filled by a smaller part of the scene, so the image will appear magnified.

Maybe this diagram will help to explain:

[johnf3f]

The answer is actually very simple. The image will be exactly the same size as it is determined by the lens not the size of the sensor.
However when you look at the image on your camera's screen/computer the image will APPEAR larger with the smaller sensor as the same size image covers more of the sensor than with a Full Frame one.
Which is better depends on what you are after. The smaller sensor makes higher demands on the resolution of the lens, whereas the (normally better) full frame sensor will require cropping to obtain the same field of view. The results will depend on the cameras in question and the quality of the lens (which I am not familiar with).
The only solution is to try before you buy. Remember to bring along a couple of your other lenses as Full Frame has distinct advantages for some uses eg landscape/wide angle.
Hope this helps.

[Ian Gray]

thanks for taking the time to answer. It's as I've always thought, but some of the things I've been reading about the new sony translucent mirror SLT cameras put a doubt in the back of my mind.

Ian

[The_Moaner]

I sold All my Sony gear not so long ago...A700 and loads of stuff and bought a Canon 7D. Now after reading reviews about the DSLT A77 I'm begining to wonder if I did a wise thing. This A77 sounds a very good camera.
I'm waiting to see images from it and if the noise is less than all other cameras.
Mind you, I'm Happy with my Canon 7D.