Understanding lenses.


Link Posted 12/08/2010 - 18:37
I currently have the 18-55mm and 55-200mm and I'm wondering if fellow Pentaxians can explain a few things to me. On the 18-55mm lens the following are inscribed on it "DAL F3.5 - 5.6 AL" and "DAL f4-5.6 ED" on the 50-200mm lens.

Looking in the manual, it just says DAL lenses do not have ultrasonic motors, but is this needed since the bit that controls the lens is inbuilt into the camera (Unlike Canon and Nikon right?)

The "ED" & "AL" means what exactly, and can someone explain this whole "f stop" to me? I have seen it thrown around a few times, and to be honest I have no idea what it means.

How does all of these have an impact on the image I take and my shot etc, also if I am buying lenses, what are the things to consider.

Apologies if my post is everywhere, the whole DSLR thing can be a bit overwhelming. Thanks.

I am also looking at the 70-300, it appears to be going for cheap. Is there anything I need to be aware of etc? Thanks.
Last Edited by ElRazur on 12/08/2010 - 18:42


Link Posted 12/08/2010 - 19:05
No, they do not need the drive motors as they use the auto focus drive built into the camera body (screw drive, as it's sometimes called).

Taking the lens designations apart:

DA - Pentax use this to designate their digital lenses - previously they have made K, M, A, F, and FA lenses

L - lightweight - these are the kit lenses with plastic mounts and no quick shift, but identical optics to the normal DA lenses

f3-5-5.6 means that the lens will open up to f3.5 at the 18mm end, but only f5.6 at the 55mm end of the zoom range. If you put the camera into Av mode, set the lens to 18mm, set the aperture to f3.5 and then slowly turn the zoom ring you'll see the indicated aperture on the LCD change as you zoom in.

"f stop" as in f3.5 is a measure of aperture. Basically the smaller the aperture the larger the number, and the less light passes through the lens. However, with larger apertures your depth of field (how in-focus things in front of or behind your subject are) will be shallower - useful for portraits where you want to isolate the person from the background, not so much for macro photography where you want the whole subject in focus. You need to think in terms of balancing the desired depth of field with the available light and shutter speed - say if you're shooting hand held in low light then you will often have to settle for a wider aperture in order to use a high enough shutter speed to avoid camera shake.

Shooting the Welsh Wilderness with K-m, KX, MX, ME Super and assorted lenses.
Last Edited by Dangermouse on 12/08/2010 - 19:06



Link Posted 12/08/2010 - 20:29
Thanks man, that cleared a few things up. What about the "ED" affix?


Link Posted 12/08/2010 - 20:49
I believe ED stands for Extra low Dispersion, & basically means that colour fringing should be well corrected.
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Link Posted 13/08/2010 - 08:19
The DA has a different lens body to the DAL.

The DA is your standard Pentax 'designed for digital' lens, and comes with a metal mount for the camera. It is also supplied with a lens hood. So, 'D' for digital plus 'A' for automatic (aperture and focus) equals 'DA'.

The DAL on the other hand has a lightweight plastic mount for the camera, and no lens hood supplied. The 'L' in the DAL stands for 'light', as in lightweight. I understand that a separately-purchased lens hood is able to be fitted, should you wish.

The 'AL' suffix (DAL F3.5 - 5.6 AL ) stands for 'Aspherical Lens'. All this means is that one of the individual lens elements inside your 18-55 has a surface that is of an aspherical design and shape. Lenses with aspherical elements do have certain characteristics, but these may not have a significant impact on how you take pictures for the time being.

All this nomenclature stuff is not as important as you getting out there and understanding how it works for you in the field! It's about taking pictures.
Last Edited by i-Berg on 13/08/2010 - 08:20



Link Posted 13/08/2010 - 08:49
Thanks man. Been reading a lot of stuff - Manual and "The digital photography book" by Scott Kelly (Vo1 1 and 2). I will be be doing some shooting today or so, and posting my findings.


Link Posted 13/08/2010 - 11:53
What I will say:

The manual is only important when you want to use a specific feature. Other than that if you switch the camera to Av mode and play around you'll learn a lot. Try putting the camera on a tripod or other stable platform then shooting the same subject close up at different apertures, this will demonstrate depth of field very quickly. I find Lego very effective for playing with macro as the studs are precisely spaced, so you can see how far in front of or behind your focus point things remain sharp.

Shooting the Welsh Wilderness with K-m, KX, MX, ME Super and assorted lenses.



Link Posted 13/08/2010 - 17:12
Once had the intention off putting them all down on paper, I think I got most of them link

ED Glass - extra-low dispersion glass Reduce the amount that light disperse when leaving the glass thereby reducing chromatic aberrations.

AL - Aspherical Lenses - a lens whose surfaces profile is neither a proportion of a sphere nor of a circular cylinder, is used to decrease various optical aberrations

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Last Edited by Anvh on 13/08/2010 - 17:15



Link Posted 13/08/2010 - 17:49
As nobody has answered your final query about the 70-300mm...

I think we need to know what brand it is. Pentax made an FA-J 75-300mm which is well thought of, although it has now been replaced by the DA (or DA L) 55-300mm. I use an elderly Pentax-F 100-300mm which I think was probably the first autofocus lens they offered with this focal length, it's not bad but a bit heavy and probably not as sharp as more recent efforts. I may replace it with the 55-300mm one day, but as I seldom use it I'm in no hurry to.

Shooting the Welsh Wilderness with K-m, KX, MX, ME Super and assorted lenses.


Link Posted 13/08/2010 - 18:11
Thanks guys.

The lens in question is made by Tamron. I have been offered one for 100GBP.

By the way, I have been out today to play around with the camera in the Av mode. I have a couple of shots posted, nothing fancy but to familiarise myself with the DoF.

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Link Posted 14/08/2010 - 16:52
Another lens related question guys.

I was reading that the sweet spot or the sharpest aperture is usually two F-stops from the lowest number? I.e if Lens if F/2.8 then sharpest aperture will be F/5.6 - F/8.

Is this applicable to the 18-55mm and 50-200mm kit lens that I have with the pentax kx?

Finally, any advice on the Tamron 70-300mm lens?


Link Posted 14/08/2010 - 23:03
That is a good rule of thumb but is not always true, nor even practical. With slow zooms I'd be shooting as fast as possible all the time, knowing my resolutions might suffer but that I needed the light. With a fast prime I'd be shooting so I had the DOF I wanted. It is rather infrequently I would say "a-ha this aperture has absolutely the best resolution so it's the perfect one for me."
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Link Posted 14/08/2010 - 23:04
The Tamron 70-300 is a pretty damn good lens. Very sharp for its class, and incredible value for money. The one issue is fairly heavy purple fringing in high contrast situations (e.g. dark object against a bright white sky), but in my experience it isn't a major problem. If you are into bird shots it might be an issue.

It has a pretty good 1:2 macro function as well.

I would however say that 100 isn't a brilliant deal for a used copy of this lens. It's not a rip off, but not a great price either.
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Last Edited by thoughton on 14/08/2010 - 23:05



Link Posted 14/08/2010 - 23:23
60 is more like the s/hand price for the Tamron. I'd wait and see if the 50-200 kit lens - nice and compact and optically very good - was enough.


Link Posted 15/08/2010 - 00:30
If you're interested in some of the more technical lens issues & specifications, this link is a good spot for one perspective (mind you it's not the 'be all and end all', unless you're just shooting test shots )

The suggestion to use the 50-200 to its fullest potential before going out and buying more glass is a good one - you might find that your style of shooting calls for glass of an entirely different (ie shorter) focal length!

The way to better understand the focal lengths you use in practice, is to use a programme like this link to analyse all of your images, as stored on your hard drive. You might be surprised at what this tells you about the settings you use.
Last Edited by i-Berg on 15/08/2010 - 00:34

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