Lens overlap

Another beginners question. Camera on tripod, light perfect as usual: If I take a photo say using a 18-55mm lens set at 18mm and another from the same position same camera etc with say 10-20mm or 10-24mm or 12-24mm set at 18mm assuming say f5.6 would the image from each be the same? Or does it not work like that? Should I get out more

Regards

Any lens set at 18mm is an 18mm lens. However, it depends on how accurate the barrel markings are, so there could be minor variations. But in essence, they will be one and the same, with perhaps varying quality depending on the individual lens.

An exception would be the 10-17mm fisheye zoom, where 18mm would look very different.

What John says is right. In principle, 18mm at f5.6 is 18mm at f5.6 (if it really is set to 18mm). However, here are a few reasons why things will be different:

-Any distortion the two lenses have is likely to be different - in particular, for the 18-55, 18 is at its widest angle, whereas a 12-24 is bang in the middle of its range. This means that you are likely to have a less distorted image on the 12-24.

-Different lenses are likely to have different sharpness at the same settings - so one image may be flat-out sharper than the other, and additionally one may have better edge-to-edge sharpness. On top of this, differences in field curvatures may contribute here. Similar factors apply to contrast, and any abberations.

-Exposure is likely to be different as even if you set the shutter speed manually (you don't mention it in the question), different lenses transmit/absorb different amounts of light. They may also give a different colour tint to the image.

-Light conditions may change between the photos (e.g. clouds moving over) giving a different "look" - and auto WB might choose different settings for the different lenses/conditions.

-You'll probably knock the tripod/get dust on the sensor when changing lenses.