Lens quality: normal lenses

This document follows on from the discussion of wideb angle lenses in the original lens quality document to discuss normal (i.e. 50mm focal lenght) lenses.

As mentioned in that document, the purpose is to try and see what difference the higher quality lenses makes in fairly ordinary picture taking situations. Will we be able to see any difference? This is not a serious scientific study with MTF curves and such. While such studies are useful, the philosopy here is that it is ultimately the images that matter.

As before, the enlargement are taken from 3,072×2,048 pixel scans by Kodak PhotoCD. In this study we examine the EF 50mm f1.4 USM, EF 50mm f1.8 II, and the EF 28-135 f3.5-5.6 IS USM lenses from Canon. Images were taken with my EOS 3 with a heavy Manfrotto tripod on Fujichrome Provia 100F (RDP III) and developed and scanned by Kodak UK.

My apologies that all the images in this section are kind of bad. Maybe I'll replace them in the summer.

The Prime Lenses

Here we compare the following two lenses:

Name	Construction.	Blades	Size	Weight	UK list price1
EF 50mm f1.4 USM	7 elem. 6 groups	8	74×50.5mm	290g	£350
EF 50mm f1.8 II	6 elem. 5 groups	5	68×41mm	130g	£90
1. The UK list prices are from December 1999.

Sorry: no 'L' series lens this time! The EF 50mm f1.0L USM weighs in at almost 1 kg and is much more than I need. With a list price of £2,200 it is also much more than I can afford. However, the f1.4 lens is usually considered a pro lens, and it is almost four times more expensive than the f1.8. Is it really worth it?

First I tried both lenses at f8. I can see no differences at this aperture, and I wasn't really expecting any significant improvement in using the more expensive lens. At this aperture they are two fine lenses.

However, one of the advantages of the larger maximum aperture (lower f-number) is that at moderate to large apertures the expensive lens is already stoped down which should improve image quality. If we are to see a difference, then it is when the lenses are very open.

As an example, I have taken two identical images at f1.8. This is the maximum aperture for one lens while the other is stoped down only 2/3 stop. Does the expensive lens provide better image quality when very open?

EF 50mm f1.8 II, 1/2000 at f1.8	EF 50mm f1.4 USM, 1/2000 at f1.8

Even at this resolution I can see a difference between the two images. If we enlarge a bit, then it becomes very obvious:

EF 50mm f1.8 II, 1/2000 at f1.8	EF 50mm f1.4 USM, 1/2000 at f1.8

The expensive lens is much sharper than the cheaper consumer version and the image shows much more detail. The enlarged portion measures about 5.0×6.3 mm on the original transparency.

So which one should you buy? If you are from the f8 and be there school and take most or all of your images at a mid aperture, then you can happily go for the f1.8 lens. However, if you ever take pictures at large apertures then I feel the f1.4 is easily worth the money. Your milage may vary: I hope these examples give you a basis on which to start and form your own opinions.

Primes versus Zooms

If you can not see any difference at f8 between the two prime lenses, what happens when you compare it to a zoom at this aperture? We have seen that it makes a difference with very cheap zooms and very expensive primes (i.e. fixed focal length lenses). What happens when we compare the moderately priced EF 50mm f1.4 USM with a mid-range zoom like the 28-135 IS:

Name	Construction.	Blades	Size	Weight	UK list price1
EF 50mm f1.4 USM	7 elem. 6 groups	8	74×50.5mm	290g	£350
EF 28-135mm f3.5-5.6 IS USM	16 elem. 12 groups	6	78×97mm	540g	£500
1. The UK list prices are from December 1999.

At normal sizes there is not much difference:

EF 50mm f1.4 USM, 1/100 at f8	EF 28-135mm f3.5-5.6 IS USM at ~50mm, 1/100 at f8

Let's zoom in a bit to see better:

EF 50mm f1.4 USM, 1/100 at f8	EF 28-135mm f3.5-5.6 IS USM at ~50mm, 1/100 at f8

There is a difference, but it is rather small. Now you know one reason why they say f8 and be there: your lens is probably very sharp at or around this aperture. (The other reason has to do with depth of field.)

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