[Roadboat24]

In a "Lively" discussion, the Pentax 28-105mm f3.5-5.6 was a great versatile lens. Which it is, But. I think a 24-70mm f2.8 on a Pentax K1 is more so. On the short end, in full frame, I get 24mm @f2.8. And at the long end, in crop mode, I get, essentially, 105mm at f2.8..... I'm I wrong?

[photoptimist]

It all depends on what you shoot. The 105 end is much more useful for people shots than is the 24 end. But the 24 end is much more useful for landscape than is the 105 end. The f/2.8 aperture is much more versatile for low-light but the 3.75X zoom range is more versatile for tourist/walking around scenarios.

[Pentax Syntax]

You are not wrong but the topic is complex. Focal length is 70 on both crop and full frame. Because of the crop, the images have roughly the same FOV as an 105 lens might have on full frame. The maximum aperture is still 2.8 on the crop camera as you state. However the effect of that aperture will be different on the crop sensor. Because the distance to your subject must change to obtain the same FOV on FF and crop, the effect of the 2.8 aperture will differ in terms of perceived depth of focus and background blur. Lots of controversy associated with this topic 😊

[ZombieArmy]

From a cost perspective alone, for most people I'd imagine a 28-105 + wide angle prime would be a suitable solution. The 24-70 is very expensive.

[Papa_Joe]

On the short end you are right. You can not get a 24 mm wide picture out of a shoot with 28 mm. But you can always crop a picture, you even don't need a crop mode on the camera, you just crop the image file. Or to follow you line of argumentation, you will always get 158 mm at f3.5-5.6 out of the 28 - 105 in crop mode.

Conclusion: Always base your decision on the physical features of a lens.


I may add the following to what photoptimist said: For portrait I would stick with the 24-70. The 105 mm are nice, when shooting people without being noticed. For portrait I would prefer 70 mm at 2.8 which give a much shallower DOF, then 5.6 at 105 mm.


Else keep in mind what photoptimist said. It is still a tough decision. You will always need an additional focal range i.e. lens. To be honest, I think no one can give you a really good advice for practical usage between these two without knowing the subjects you prefer and the experience you already have. Sorry .

[DeadJohn]

...And at the long end, in crop mode, I get, essentially, 105mm at f2.8..... I'm I wrong?

In crop mode you are discarding half the megapixels you paid for. For fair comparison between lenses, in crop mode the 28-105 gives the same field of view as 158mm.

The 24-70 and 28-105 are both good lenses. If you need f2.8 or the 24-28mm range, buy the 24-70. If you need more than 70mm, prefer a less bulky lens, or want to save money, buy the 28-105.

In my experience, mostly landscapes and general photography, rarely portraits, the 28-105 works best for me. I'm usually shooting landscapes at f8 through f/16 so I don't want to carry the extra weight of f2.8. For wider than 28mm I have other lenses.

[Roadboat24]

So. Let me restate. Just so I'm clear.
Would not the image from 105mm @f2.8 on a full frame body be virtually identical to 70mm @ f2.8 on a 1.5x crop body?

I'm thinking it would be.
I'm also thinking (my preference) I'd rather have a f2.8 lens vs a f3.5-5.6 lens.

Thanks everyone, I appreciate your enlightenment

[bm75]

Would not the image from 105mm @f2.8 on a full frame body be virtually identical to 70mm @ f2.8 on a 1.5x crop body?

No it wouldn't. Focal lenght is different, camera is different so image is different. Field of view can be the same. Just try this: put your FF lens on a FF body, shot an image , say @105 mm ISO 100 @f/4 whatever the speed for a correct exposure. Then put the DA70 on the same FF body, shoot in APSC mode at the same values . Is the image the same? no....it's a different image. Lenses have their optical specifications and rendering depending upon optical schemes and, obviously, FL.

[Roadboat24]

Ok. bm75. I did just that. 100mm f2.8 on full frame, 24-70mm set at 66.. f2.8 on crop. Yes FOV is the same, but I see your point DOF is a bit different. Good to know. Thanks.

[awscreo]

On the short end you are right. You can not get a 24 mm wide picture out of a shoot with 28 mm. But you can always crop a picture, you even don't need a crop mode on the camera, you just crop the image file. Or to follow you line of argumentation, you will always get 158 mm at f3.5-5.6 out of the 28 - 105 in crop mode.

Conclusion: Always base your decision on the physical features of a lens.


I may add the following to what photoptimist said: For portrait I would stick with the 24-70. The 105 mm are nice, when shooting people without being noticed. For portrait I would prefer 70 mm at 2.8 which give a much shallower DOF, then 5.6 at 105 mm.


Else keep in mind what photoptimist said. It is still a tough decision. You will always need an additional focal range i.e. lens. To be honest, I think no one can give you a really good advice for practical usage between these two without knowing the subjects you prefer and the experience you already have. Sorry .

You can stitch 2 2mm shots)

[Roadboat24]

So my conclusion is, the depth of field will remain the same for any particular focal length, irregardless of field of view.

[clickclick]

So my conclusion is, the depth of field will remain the same for any particular focal length, irregardless of field of view.

As long as the distance is constant, because as you vary the distance to/from the subject, the DOF will become less/more.

[photoptimist]

So. Let me restate. Just so I'm clear.
Would not the image from 105mm @f2.8 on a full frame body be virtually identical to 70mm @ f2.8 on a 1.5x crop body?

I'm thinking it would be.
I'm also thinking (my preference) I'd rather have a f2.8 lens vs a f3.5-5.6 lens.

Thanks everyone, I appreciate your enlightenment

You are right. Viewed at the whole picture level and modest print sizes, a 105 mm @ f/4.2 on a K-1 would replicate the both the field of view and DoF of a 70mm @ f2.8 on the K-1 in crop mode. Of course, the 105 on K-1 FF image could be printed larger than the 70 on K-1 crop mode image could. And the 105 could be used in K-1 crop mode to get 157 mm shot that the 24-70 could only provide as a 7 Mpix crop.


It's also worth noting the 24-70 is about double the price and weight of 28-105. As far as the wallet and camera bag are concerned, one could get a 28-105 and another smallish lens for the price and weight of the 24-70. (I'm reminded of a marketing poster at General Dynamics where they made single-engine F-16 jet fighters. Their response to the larger twin-engine F-15 was a poster with two F-16s and the tag line "The Best Twin Engine Fighter in the World").

[lesmore49]

When I bought my K1 I wanted to buy either the 24-70 F 2.8 or the 28-105 Pentax lenses. I wrestled with the decision and did read , check out photos, etc...about the performance of each lens.The advantage of the 24-70 seemed to be mostly the fact that it was an F 2.8 and that it was a wider angle. The 28-105...even though significantly cheaper...and of course just a mere 'kit' lens (but what a 'kit' lens) ..... seemed to have either fairly equal ...or in some tests...better photo quality....according to what I read and my judgement on photos taken with each lens, that I saw mostly in this forum.

I had the budget for either lens, so that wasn't a big consideration. I chose the 28-105 and have been happy. Part of my decision making was influenced by the fact that I also have a FA 50mm F1.4, a 50mm D FA Macro F 2.8 and 40 and 70 Limited(s). I also now have a D FA Pentax Macro F 2.8 100mm which I think will be a wonderful portrait lens. All these lenses are ...in my experience...fast. The 50 normal an F 1.4, the 70 Ltd...an F 2.4...the rest as fast as the 24-70 @ 2.8. They are also all primes...they might have a bit of an edge over the zoom 24-70...in portraits.

The negative of course is that the primes don't have that convenient zoom factor where I can stand still and let the zooming in and out function save me the effort of moving my 'bod' back and forth. But frankly, I don't mind doing the old two step forward and two step backwards with my primes.

In the end, even though I know the 24-70 is one of trio (add the 70-200...and the 15-30 for a complete set) that every photographer should aspire to...I find for my purposes...the 28-105 as a general purpose walk around (does fairly good portrait work) and a whole stack of quality primes...works well for me.

[Roadboat24]

This whole thing came about because the discussion was to get the 28-105. But I already have the 24-70 and 70-200. I couldn't see what advantage to now include the 28-105 And as I could put the K1 in crop mode and achieve 105 without changing lenses. Anyways, That's what I do..for better or worse. Yep, they are both Tanks, I don't mind.