Re: Equivalent crop sensor lenses

Peabody wrote:
Ofnuts says...

> On the P&S, however, specs (especially the ones
> displayed in the store) are usually 35mm equivalent
> (because it gives bigger numbers) and the bridge camera
> sporting a 28-420mm zoom has actully got a 5-72mm lens.

Well, this must vary a good bit. My Canon A590 P&S shows
5.8-23.2mm on the lens, and you have to go to the Specs
page at the back of the manual to find the 35mm equivalent
(35-140).

The more useful way would be to state it like 0.5x or 2x relative to the normal view for that sensor where 'normal' is 1x. Nobody ever does that* so this is pretty much an academic discussion for better understanding of the concepts.

You do see numbers like 4x or 15x used for zooms but that's different and tells the zoom range relative to nothing but it's range: no clue how wide or long in absolute terms. Using your P&S example above 140/35mm (or 23.2/5.8mm) is a 4x zoom range.

Now, if we call 50mm eq normal, then 35mm eq is 0.7x (35/50) and 140mm eq is about 3x (140/50). So we would call it a 0.7x-3x zoom in my proposed terminology. The 28-420mm eq bridge camera quoted above is a 15x zoom with a range of about 0.6x-8x in my terminology.

Teleconverters would fit into this scheme nicely. Take a P&S zoomed to normal focal length of 1x and put a 2x converter on and you have a 2x zoom. Put a 0.7x wide angle converter on and it's a 0.7x zoom.

There is another can of worms regarding what 'normal' means. The simple answer is that a normal field of view is focal length equal to the diagonal of the sensor's dimensions. Whether that has real-world relevance or not, every technically trained photographer knows what a normal lens is for their format; for FF SLR, medium format or large format view camera on 8x10-inch negatives. In fact, 'normal' for FF SLR is closer to 43mm than 50mm, and that's probably just a meaningless tradition but it's still in the ballpark.

* 'nobody ever does that' -footnote
The standard for microscopes, binoculars & telescopes is very close to this idea but it's relative to the size you see with bare eyes. Cameras aren't about seeing with your bare eyes; but about looking at a print; so the base point is different. Various camera designs could have all sorts of different viewfinder magnifications but still produce the same field of view in a print using a normal lens. For my particular FF DSLR, a 75mm lens gives a 1x 'telescope magnification' through the viewfinder, but that really isn't relevant.

The other place you see these 1x type descriptors is in macrophotography and that has yet another meaning. There, it means the subject is enlarged to the life size on the sensor at 1x, also stated as 1:1. This is as meaningless as absolute focal length in mm given different sensor sizes but it's simple math and is the traditional way to express magnification for closeup work. Focal length has completely different relevance for macrophotography, where magnification on the print is the more relevant metric. A 35mm macro lens on a bellows with lots of extension gives about 5x magnification on the sensor but my system would call it a .7x at infinity focus, which is the opposite effect. These are completely different things.

If we wanted a universal standard for macrophotography magnification, it would make sense to say 1x is life size on an 8x10 print, since cameras are all about prints, not what you see with your eyes through the viewfinder. DOF is traditionally calculated with an 8x10 print as the base point. A 1:1 macro lens on an APS cropped DSLR covers a 24mm wide subject on the 24mm wide sensor. 24mm is about an inch, so an 8x10 print is about 240mm wide on the long side. So my macro magnification standard would call this a 10x magnification. On FF the same lens would provide about 7x by my new definition for standardized magnification in macrophotography.

An even better way to measure macrophotography magnification might be to account for pixels. 300px/in is a common standard for prints, so to print at that resolution it would take about a 7MP sensor to make an 8x10 print at life size on the print. Call that 1x magnification. Now try calculating the magnification for a 1:1 macro lens on a 12MP FF sensor and you'll see it requires a lot of math. That's why nobody does it this way :-) And why traditionally, photographers simply refer to the focal length in mm assuming they are talking with someone familiar with the format in question.


--
Paul Furman
www.edgehill.net
www.baynatives.com

all google groups messages filtered due to spam
.

Relevant Pages

  • Re: Best Camera for NIght Shots?
    ... don't forget to compare available zoom lens ranges ... with available f/stops for those lenses. ... ISO with a larger sensor. ...
    (rec.photo.digital)
  • Re: Something I dont get about zoom lenses.
    ... The zoom ratio is the ratio between the longest and shortest focal ... This is not magnification, it is the zoom ... camera or lens manufacturer represents zoom ...
    (rec.photo.digital)
  • Re: Something I dont get about zoom lenses.
    ... What do you think is a 40x zoom? ... For example for the 18-200mm lens you spoke of, ... Magnification for a crop sensor camera, where a 35mm lens might be ... To get 40x magnification on a crop sensor camera you would need a lens ...
    (rec.photo.digital)
  • Re: Zoom and magnify relation
    ... The zoom factor is simply the longest focal length divided by the ... shortest focal lenght of a zoom lens. ... The magnification on the other hand depends on the focal ... length and the size of the sensor. ...
    (rec.photo.digital)
  • Re: Extension rings for macro
    ... compositions/perspectives you'd get with a 100mm lens on FF. ... Due to the smaller sensor, ... full frame equivalence as far as final image size is concerned. ... But the final image magnification is not the same; ...
    (rec.photo.digital.slr-systems)