[jjfdvm72]

I realized that a very important factor when comparing zoom lenses is their speed (f/stop). My question is, how do you compare the "speed" of the different lenses, when they only list the smallest and largest appertures and they have different zoom ranges??

I can't afford a fixed f/stop lens (trying to stay <$500). I was wondering about purchasing the Tamron 18-250 f/3.5-6.3, but now I am wondering if I would be better with a more limited range zoom (ie: SIGMA/TAMRON 70-300 f/4-5.6 or Pentax 55-300 f/4-5.8) IF they do offer faster speeds (lower f/stops) at the same focal distances.

I just have the packaged 18-55mm Pentax, but I did think about perhaps buying a SIGMA 17-70mm f/2.8-4.5 if I end up buying one of the zooms that only has telephoto range. I would not be buying that 17-70 until WAY down the road, so for a while I would be using the 18-55mm for wide angle and "missing" the 55mm-70mm range (if I picked the SIGMA/TAMRON)

So the options are:

1) Buy Tamron 18-250 and use it as the all inclusive lens and perhaps sell the 18-55mm

OR

2) Buy one of the telephoto zooms (if they would allow faster speeds than the Tamron) and try to save up to eventually (couple of years) by a SIGMA 17-70 (or comparable) to substitute my 18-55mm (and improve overall speed)

Does the convenience of the one lens 18-250mm beat the possible inc speed and price of the 70-300mm?

Sorry for the long post...

[Canada_Rockies]

The well reviewed Pentax DA 16-45 f/4 has a constant f/stop, and goes quite a bit wider than your kit lens, and is about $400, sometimes less. This moves the range into the wide area, so if you are looking for longer, it won't do.

The Pentax/Tamron 18-250 reviews very well indeed, and might be a good lens for you. I would be tempted, and was, to save up for the DA* 50-135. I realize it isn't going to be out in the 250-400 territory, and I have that range covered with manual focus lenses, myself.

I don't think that if I were the owner of the 18-55, and bought the 18-250 that I would sell off the 18-55. A lighter package for indoor work would be very handy, and the kit lens sells for so little.

You can see the direction my kit is going in my signature. I have 12 - 135 mm covered with top quality replacement lenses, and will gradually as finances allow extend toward the long end.

It has taken me a long time to build my system, and it is not yet complete, and probably will never by complete, but it is getting closer to what I need as time goes on. I have added lenses as I grew with the system, slowly over 47 or 48 years.

I would seriously think before buying a slow long focus lens. As the lens magnifies the image more, it also magnifies all the imperfections in my technique. SR helps, but does not resolve all the problems.

[Lowell Goudge]

For tele zooms I consider a reasoable zoom tele has a 77mm filter.

this equates to 200mm at F2.8, 300mm at F4 and 400mm at F5.6

Any slower than this and for wildlife I think you are extremely limited.

for short (i.e. wide) zooms you cant equate them this way because the filters become very large just to get out of the field of view.

Having said that, any zoom, at any length with F2.8 is fast.

I like constant apature better than variable apature, but this is hard to avoid in ultra wide.

[navcom]

I have a Pentax DA 16-45mm and also a Tamron 70-300mm f/4. I shoot mostly landscapes, so I usually have the 16-45 attached to my body. I use the Tamron for those times I just need to get in a little closer. It's rare I go all the way to 300mm. I am very happy with both lenses and don't need something that covers the whole range for speed.

I guess it all comes down to what you are shooting and what your needs are going to be. Personally I agree with Canada Rockies...don't buy a slow long lens. Since you don't have the finances for the speed, go with the 18-250mm...either Tamron or Pentax. They are both exceptional lenses for the money. And don't part with your kit lens.

The one thing I don't like about the Tamron or Pentax 18-250 is the tendency of the lens to accidentally extend by gravity when you point the camera down. They have a lock to prevent this but I know I would inevitably forget at some point and it would extend...most likely right into something blunt and hard. :-) If you do buy one, a UV filter is a must.

[Lowell Goudge]

The tamron 28-75 also suffers from zoom creep, as does my vivitar series 1 70-210 F3.5 (circa 1981) zoom creep is an old issue.

I wouldn,t worry about it.

as for lens speed, my comments from above hold true, I prefere fixed apature to variable,

[jjfdvm72]

Lowell Goudge -

Does the filter size equate to the apperture of the lens? I know the apperture is measured inside the lens, but your comment seemed to equate the larger the front of the lens, the bigger the apperture (which would make sense to me...)

[Lowell Goudge]

Lowell Goudge -

Does the filter size equate to the apperture of the lens? I know the apperture is measured inside the lens, but your comment seemed to equate the larger the front of the lens, the bigger the apperture (which would make sense to me...)

Not directly on all lenses, that is why I qualified it to telephotos.

Apature (i.e.f2.8, F4 etc) is a mathematical ratio of focal length over diameter.

On telephoto's the filter size is the limiting factor. for example a 200mm lens with a 77mm filter can be AT BEST F2.6 but only if the opening is 77mm. In fact the maximum opening at the front of the lens is a little smaller, hence F2.8. This works well above 100mm, My 135 F2.5 has a 50mm filter, the math suggests maximum apature of F2.33

On shorter focal lengths as field of view increases, the dependance on apature and filter diverge, because the finter diameter is the size it needs to be to avoid vignetting and is usually very large in diameter relitive to what is needed for the F stop of the lens.

I use the filter size on tele lenses as a way of defining the light gathering capability. for example, my 300mmF4 has a 77mm filter, and I can add the 1.7x AF TC to it to get 510mm. This works very well for me. If someone suggests that I get a BIGMA, the filter is only86mm in diameter. and in reality is so close to my combo in size the difference is less than 1/2 stop. My own view is that it is not worth it to me to get the BIGMA for a 1/2 stop benefit at 500mm. Now if it were F4.5 (112mm filter) I would be interested.

[jjfdvm72]

So I guess in summary, I am not really going to see a big difference in the "speed" of the lens if I am comparing the Tamron 18-250, sigma/tamron 70-300 or the pentax 55-300. Correct?

If that is the case, i might as well opt for the Tamron 18-250 and get the wide range in the zoom for the convenience factor of not carrying around multiple lenses. Anyone disagree?

[drabina]

Well, it depends. Currently I have the kit lens (18-55mm) and Sigma 70-300mm zoom. To get a bit better (faster) zoom, I am replacing the kit lens with Sigma 17-70mm. But I know that 17-70mm would cover about 95% of my needs just because I used to have film camera with 28-105mm lens (basically 35mm equivalent of 17-70mm) and I loved it (not so much the quality as the range). I know the temptation of the all-in-one zoom lens. It is really hard to get the perfect fit without trying few lenses first.

[Lowell Goudge]

So I guess in summary, I am not really going to see a big difference in the "speed" of the lens if I am comparing the Tamron 18-250, sigma/tamron 70-300 or the pentax 55-300. Correct?

If that is the case, i might as well opt for the Tamron 18-250 and get the wide range in the zoom for the convenience factor of not carrying around multiple lenses. Anyone disagree?

the tamron is the slowest of the bunch, by 1/2 stop, but notice also that it is not as long 250mm vs 300mm.

You might find that at 250mm the other 2 lenses are close to a stop faster (since they are variable apature lenses) and this means 2 times the shutter speed wide open. That 1 stop difference is A LOT.

That is why I look for really big and fast lenses, my 300mm is F4, which is another stop faster.

the net result is I get to shoot at close to 4 times the shutter speed than the 18-250 zoomed all the way in, Shake reduction or not, shutter speed does matter. the down side is weight, you carry pounds extra for the speed. It's your back. Mine is fine, so I go for all the speed I can get

[jjfdvm72]

I'm confused. when you say 2 stops faster, are you talking about the f/4 vs f/6.3? wouldn't 2 stops less opening form f/4 be f/8? (ie f/4-f/5.6-f/8) I thought that the Tamron 18-250 at f/6.3 compared to the Sigma 70-300 at f/5.6 would be more like a 1/3 stop difference. If so, would that even change the speed the camera would need to shoot at to get the proper exposure?

They way I was looking at it (incorrectly probably) was that I would need the same shutter speed to get proper exposure whether I used f/6.3 or f/5.6 due to only being 1/3 stop difference. If that's so, is it even worth it to get that extra 1/3 stop?

[little laker]

The Sigma 28 - 70 f2.8 appeared to be a nice lens when I tried it, and it should be within your price range.

I chose the Sigma 24 - 70 f2.8 instead, but it's suggested retail value is just above your limit. You can probably find it for less though.

[egordon99]

Stops go f/1, f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16.......
So f/6.3 is less than a full stop slower than f/5.6. BUT it's still slower, so if you used the same shutter speed at f/5.6 and f/6.3, the f/6.3 will be darker.