What is internal focus AND is it bad that lens doesn't have it?

I am thinking of getting the 28-135 mm 3.5/5/6 cannon lens. I

ordered a 100 mm (macro) lens and a 35mm lens BUT realize I want the

other features in between, but noticed the specs say it does not

have internal focus. What is this and why it is important (or not)?

 

ALso, has anyone had experience with the cannon EF 28 - 200

3.5/5.6 ...it is affordable and I like that it goes to 200, but

wonder how good it is.

 

Thanks as always.

Basically, lenses focus closer as all the elements move forward together as a unit. This is called linear focusing. Another method of focusing is when certain internal element or elements move and the overall length of the lens does not change. Actually, there are internal and rear focusing lenses. The rear focusing lenses move an element or elements at the rear of the lens, as you would suppose.

 

Internal focusing lenses are not necessarily better than linear focusing lenses in terms of accomplishing the job, but may focus faster. Linear focusing lenses may develop more play in their mechanism which racks the lens in and out whereas internal focusing lenses may be more convenient as you don't have to rack them back to the infinity position for more compact storage.

 

However, linear focusing lenses usually have fewer elements; classic lens formulas such as Tessars fall into this group.

BTW, Pamela, the original Canon EF100mm macro lens featured linear focusing whereas the newer (present model) has internal focusing. Also, there are also zoom lenses which have internal zooming and others, such as the 28-135 which gets longer as you zoom.

 

I'm afraid I can't help you on the subject of the 28-200 lens but I'm sure someone else will.

Intenal focus (IF)lens auto focus faster because the rear elements are smaller (lighter) and need less travel. However, I assume you order the EF35/f2, that lens is small and light so it has about the same quickness as an IF lens. Canon also mostly matches ring USM motor with (faster and quieter) IF so they are in general also make less noise. I have a EF35/2, it has great preformance. The current EF100mm macro has IF as well as USM. Unless you get the old pre-1990 version, it should have IF. The pre-1990 version uses a non-IF design and has a different optical formular. It is generally consider as a bit less good (compared to the newer version) optically (not because of IF).

 

My 2 cents, EF28-200 is a super zoom. Super zoom is very good for snap shoot. However, it is hard to find a super zoom that compare well to the lens that you listed above. A 70-200 zoom work better for the 135-200mm function. The 28-135IS it is a good choice. I have good experience with mine.

The main benefit of internal focusing is in long lenses, 300 mm and above where the focusing can be accomplished by moving a few small lenses inside the lens, instead of the whole large lens assembly. This makes focusing much smoother and faster, whether it is done by motorised autofocus or by hand.

 

In general, zooms with very large focal length ranges are not optically good. The few exceptions of acceptable lenses include for example the Canon L series zoom but then they are both large/heavy and expensive.

According to several reviews that I have read, the Canon 28-200 is one of the poorest performing superzooms being made today. The Canon 28-135 is a good consumer zoom with the added benifit of Image Staibiliation. It's not perfect, but pretty good for the money.

<p> <i> </i> </p>

<p> <a href="http://photonotes.org/articles/beginner-faq/lenses.html#rearinternal">What is rear or internal focussing?</a> <b></b> </p>

<p> Take note that the fact that the lens become physically longer or shorter when you <b>zoom</b> or not has nothing to do with IF. </p>

 

<p>Happy shooting, <br>

Yakim.</p>

One other side effect of internal focus is that the effective focal length is reduced as you focus closer - effectively the closer focus is accompanied by zooming out. Lens focal lengths are measured at infinity focus, and there is usually a tolerance of around 5% or so around the claimed focal length. Your 100mm macro (if it is the latest Canon version) actually becomes something like a 70mm at 1:1 closest focus.

 

The 100mm macro and the 35mm are fine lenses. I'm not sure whether you are shooting with a full frame or a crop camera, but I'll assume it is the former. If you want to keep the same sort of quality in your zoom, I'd suggest you pick the Tamron 28-75 f/2.8 instead of the other lenses you are considering. That leaves your options open for covering longer focal lengths. Among choices to consider for that might be the Canon 55-200 (compact and sharp), Sigma 70-300 f/4-5.6 APO Macro DG (optically the best xx-300 below $600), Canon 70-200 f/4 L and Canon 70-300 f/4-5.6 IS (new and already shown to be a fine lens).

On a prime (non-zoom) lens Internal Focus is nice because the lens length does not change as you focus, thus you don't have to be careful as to where you put your fingers (as opposed to the 50/1.8 which is not IF, and changes length during focussing, which can be surprising if your finger is on the moving part).

 

Also the lens body can be better sealed against dust as it does not change size. Super pro zooms also have internal zoom, which permits good sealing, as opposed to the standard zoom whose pumping action will suck in air and dust everytime you use it.