Some lens questions - Canon

[ajf]

This might seem an odd question.
If i have taken a photo with for example a 70mm lens.
Is it possible to roughly compare it to an image taken with a 100mm lens by cropping the image?

Scenario:
I have a Sigma 24-70 lens. It doesn't give me the reach i need in some circumstances plus I am not happy with the quality.
My initial though was replace with Canon 24-105, but not sure now much difference the extra 35mm would really make?

On a different line, what is the general consensus on the Canon 85mm f1.8 lens?

 

[ajf]

Thanks.
How would I be able to work out the amount to crop?
By the way this is an APS-C sensor, not FF. 600d.

That Nikon lens sounds ideal to be honest. Maybe not Canon 24-70 sharp but probably a whole lot better than my Sigma
Not sure that's a justifiable reason to change body as well!
I assume this is the lens you mean:
http://www.wexphotographic.com/buy-nikon-28-300mm-f3-5-5-6-g-af-s-ed-vr-lens/p1522139

I have seen the Canon 28-300mm but yes price was an issue, plus is it actually still current?

I can lose the extra aperture as I don't use the current lens wide open except for intentional low DOF static shots, which was where i thought I could get the 85mm for.

 

[ajf]

Thanks for the additional comments.
I guess it depends on usage but not sure why the 24-105mm seems an odd choice?
The very low end of the range is not that important to me for this lens use, but the range is.
The 15-85 is less of a benefit here than the 24-105, which from what I have read is a very good lens quality wise.

When I need wider angles, I have both a 10-22 and 17-40mm lens I can use, at which point I don't also require a high top end range.

I think my ideal lens to replace the 24-70 would be something around 40-200 (or Canon 28-300)!

Actually, looking at purely this scenario buying a Nikon body and that 28-300mm lens actually almost makes sense financially when compared to the Canon 28-300.

 

[ajf]

Again thank you for all the comments and suggestions.
I think maybe I need to take stock of what to do next. Especially given the quality comments on the 24-105mm. I had thought it was quite a high quality lens.
The 70-200mm is a bit tight at the low end too!
I might try to hire one though and give it a go.

To show what I am after, these two shots are taken from roughly the same location at Prescott, both with the 24-70mm lens and both at 70mm:

Close to camera. At 70mm doesn't give much room for error as seen here. 40-50mm is about where I need.

sample_70mm_close by ajf.350d, on Flickr

Furthest from camera at some point. 70mm is OK but limits close up detail. I can crop admittedly.

sample_70mm_distant by ajf.350d, on Flickr