[MDRatliffe]

I want to get a longer useable range for a wildlife/birding lens, from my current tamron 70-300. I'm not sure how much more useable cropping I could get from a sharp 300mm lens, compared to something like the bigma. From thread's I've found, the 300mm primes, or sigma 100-300 f/4, are all going to be sharper and generally better than the 50/150-500mm sigmas, but I didn't see whether they're sharper to the point that cropping in would be comparable, or better.

[kypfer]

Unfortunately I don't have a long prime to compare it to, but my Sigma 150-500mm gives me all the cropping I need for birding with a 24Mpixel sensor.
Once you get out "so far" horizontally, depending on weather conditions etc., atmospheric distortion, especially over water, is often the over-riding factor.
Just to give an idea … on a "good day", shooting more or less vertically, an image of an over-flying jet (about 7 miles up) can reveal the registration letters under the wing, so the lens works, but shooting "across the pond" may well be limited to 300ft or less!
I've made similar enquiries in the past and replies from those who've owned (or still own) both a "bigma" and a Pentax 150-450mm suggest there's very little difference in image quality from either lens, suggesting that the "bigma" is up there with the best.

Do remember, with a long prime, actually finding your subject can be a problem. At least with a zoom you have the option to zoom out, locate your target, then zoom back in to frame and focus.
Having said that, a faster lens (f/5.6 or wider) will likely give you better tracking than the f/6.3 "bigma", but you'll be paying for the privilege!
As others will likely recommend, there's no substitute for getting closer

[marcusBMG]

You may find this test pic thread I posted of interest. I added some test pics from the sigma 100-300mm f4:

Pentax DFA 150-450mm vs Sigma DG 150-500mm APO HSM. K3-ii test pics. - PentaxForums.com

[mlag]

When you compare 300 to 500 mm lens, the angle of view changes from roughly 5.5° to 3.1° (about 56%) , so you would need some serious cropping almost halving (about 60%) the number of pixels.


i have seen lens reviews where they indicate indeed a drop in resolution between primes and zooms, but it was expressed in figures like 2600 to 2200 (lpi or so...), so the drop in quality looks less heavy to me for the zoom than the drop from the cropping.
I have seen similar discussions on Teleconvertors, but there it matters more as you are adding glass quality of two lenses, so the drop in quality might matter more than the cropping....

Another point is that the purpose of your pictures also matters. For small scale photo prints ( A5 format (halve sheet of paper) or less) it hardly matters , both the cropped and "less sharp zoom but closer tele" will probably both yield very good results. If you want large photo prints (like A3 size - 4x the size of a sheet of paper) things might become visible...

The long lenses have also other disadvantgages , the aperture allows less light (hence shutter speed is slower) and they are more prone to (handheld) shake/vibrations... so there is more risc for motion blur.


Today i tested a just acquired secondhand Sigma 50-500 F4-6.3 on a bird (quite common one but he presented himself as testsubject) - 50% crop. Resolution looks quite good to me....
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[hcc]

While I appreciate that the OP use a zoom lens, I would like to share another option.
I use a FA*300mmf4.5, and I found that the HD-DA-T1.4x TC is a great option for extra reach without loss of IQ. Pairing this prime with this TC, I have basically a 420mm lens, and I enjoy the extra reach.

PS: I have also a Tamron 2xTC, and I can notice some loss of IQ. So, the right TC is important
I hope that the comment may be useful and helpful to some....

[TwoUptons]

I have a Tamron, and it's not fantastic at 300mm... especially with the purple fringing...

I have a Pentax A-400mm f5.6 that is better, but it's manual focus and has its own issues...
The comments above about atmospheric issues are the big one for me with this lens... mainly because I live where it is humid.

I also have a Pentax 55-300 (not the PLM). It's better at 300mm than the Tamron, but it isn't as good at the really close stuff.

-Eric

[ramseybuckeye]

Years ago I rented a Sigma 50-500 and test used it for 3 days of warbler shooting, it was a good lens, but ultimately I opted for the DA*300 for the speed, most of my usage was in wooded areas without much light. I've never regretted that. about a year and a half ago I bought the DA 1.4 rear converter, which with the 300 is a pretty awesome combo. I also have the 55-300PLM which also works well with the TC in good light. So you might think about the lighting conditions in the places you normally shoot. Look at the apertures and shutter speeds on your photos you have taken and see what differences they may make.

A couple with the KP, DA*300 and TC. Of course the closer the better, the heron is uncropped, the kingfisher is heavily cropped

[MDRatliffe]

Thanks for all the advice and that comparison! I would like to get closer to my subjects, but when they're on the other side of a 30m pond, or on the side of a mountain, it's a bit challenging. I don't know which focal range will be better for me, but at least now I feel good to not worry about IQ, and actually think about how I use aperture and so on.

[Wasp]

I look at things from another point of view. If you have a 500mm, your subject is much bigger in the viewfinder. You can see what it is doing, how the light catches it and so on. Surely this will aid in taking a better photo.

[RedRuff]

I have a K1 with F*300mm for my default kit. I also have a 500mm F4P manual focus Nikon lens that I attach for shooting when I don't need infinite focus. Even though the 300 crops very nicely and is autofocus I've not regretted days where I had the extra reach. I think that might be because I'm just as likely to grab a film camera as my K1 so I guess cropping isn't really in my mind when taking photos.
The Nikon lens is really nice though...

K1+500mm


ME Super+500mm


K1 and F*300 cropped to within an inch of its life: